diff --git a/CHANGES.md b/CHANGES.md
index 1d9de3f2a6..d72933d484 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -6,6 +6,7 @@
 * Misc: yojson is no longer optional
 * Compiler: speedup global_flow/global_deadcode pass on large bytecode
 * Compiler: speedup json parsing, relying on Yojson.Raw (#1640)
+* Compiler: Decode sourcemap mappings only when necessary (#1664)
 * Compiler: make indirect call using sequence instead of using the call method
   [f.call(null, args)] becomes [(0,f)(args)]
 * Runtime: change Sys.os_type on windows (Cygwin -> Win32)
diff --git a/compiler/bin-js_of_ocaml/cmd_arg.ml b/compiler/bin-js_of_ocaml/cmd_arg.ml
index 5ffe0fd7f2..fbaf4411ce 100644
--- a/compiler/bin-js_of_ocaml/cmd_arg.ml
+++ b/compiler/bin-js_of_ocaml/cmd_arg.ml
@@ -314,7 +314,7 @@ let options =
             ; sources = []
             ; sources_content = (if sourcemap_don't_inline_content then None else Some [])
             ; names = []
-            ; mappings = []
+            ; mappings = Source_map.Mappings.empty
             } )
       else None
     in
@@ -543,7 +543,7 @@ let options_runtime_only =
             ; sources = []
             ; sources_content = (if sourcemap_don't_inline_content then None else Some [])
             ; names = []
-            ; mappings = []
+            ; mappings = Source_map.Mappings.empty
             } )
       else None
     in
diff --git a/compiler/bin-js_of_ocaml/link.ml b/compiler/bin-js_of_ocaml/link.ml
index cebb9e53d7..45c2dd0c0e 100644
--- a/compiler/bin-js_of_ocaml/link.ml
+++ b/compiler/bin-js_of_ocaml/link.ml
@@ -108,7 +108,7 @@ let options =
             ; sources = []
             ; sources_content = Some []
             ; names = []
-            ; mappings = []
+            ; mappings = Source_map.Mappings.empty
             } )
       else None
     in
diff --git a/compiler/lib/js_output.ml b/compiler/lib/js_output.ml
index 33c57f23f0..64dcdcab64 100644
--- a/compiler/lib/js_output.ml
+++ b/compiler/lib/js_output.ml
@@ -1996,8 +1996,9 @@ let program ?(accept_unnamed_var = false) f ?source_map p =
               | None -> filename
               | Some _ -> Filename.concat "/builtin" filename)
         in
+        let sm_mappings = Source_map.Mappings.decode sm.mappings in
         let mappings =
-          List.rev_append_map !temp_mappings sm.mappings ~f:(fun (pos, m) ->
+          List.rev_append_map !temp_mappings sm_mappings ~f:(fun (pos, m) ->
               let gen_line = pos.PP.p_line + 1 in
               let gen_col = pos.PP.p_col in
               match m with
@@ -2012,6 +2013,7 @@ let program ?(accept_unnamed_var = false) f ?source_map p =
                   Source_map.Gen_Ori_Name
                     { gen_line; gen_col; ori_source; ori_line; ori_col; ori_name })
         in
+        let mappings = Source_map.Mappings.encode mappings in
         Some { sm with Source_map.sources; names; sources_content; mappings }
   in
   PP.check f;
diff --git a/compiler/lib/source_map.ml b/compiler/lib/source_map.ml
index 179c9b021d..a2d30d9f85 100644
--- a/compiler/lib/source_map.ml
+++ b/compiler/lib/source_map.ml
@@ -50,7 +50,165 @@ type map =
       ; ori_name : int
       }
 
-type mapping = map list
+let gen_line = function
+  | Gen { gen_line; _ } | Gen_Ori { gen_line; _ } | Gen_Ori_Name { gen_line; _ } ->
+      gen_line
+
+let gen_col = function
+  | Gen { gen_col; _ } | Gen_Ori { gen_col; _ } | Gen_Ori_Name { gen_col; _ } -> gen_col
+
+module Mappings = struct
+  type t = Uninterpreted of string [@@unboxed]
+
+  let empty = Uninterpreted ""
+
+  let of_string : string -> t = fun s -> Uninterpreted s
+
+  let to_string : t -> string = fun (Uninterpreted s) -> s
+
+  let encode mapping =
+    let a = Array.of_list mapping in
+    let len = Array.length a in
+    Array.stable_sort
+      ~cmp:(fun t1 t2 ->
+        match compare (gen_line t1) (gen_line t2) with
+        | 0 -> compare (gen_col t1) (gen_col t2)
+        | n -> n)
+      a;
+    let buf = Buffer.create 1024 in
+    (* The binary format encodes lines starting at zero, but
+       [ori_line] and [gen_line] are 1 based. *)
+    let gen_line_r = ref 1 in
+    let gen_col_r = ref 0 in
+    let ori_source_r = ref 0 in
+    let ori_line_r = ref 1 in
+    let ori_col_r = ref 0 in
+    let ori_name_r = ref 0 in
+    let rec loop prev i =
+      if i < len
+      then
+        let c = a.(i) in
+        if i + 1 < len && gen_line c = gen_line a.(i + 1) && gen_col c = gen_col a.(i + 1)
+        then (* Only keep one source location per generated location *)
+          loop prev (i + 1)
+        else (
+          if !gen_line_r <> gen_line c
+          then (
+            assert (!gen_line_r < gen_line c);
+            for _i = !gen_line_r to gen_line c - 1 do
+              Buffer.add_char buf ';'
+            done;
+            gen_col_r := 0;
+            gen_line_r := gen_line c)
+          else if i > 0
+          then Buffer.add_char buf ',';
+          let l =
+            match c with
+            | Gen { gen_line = _; gen_col } ->
+                let res = [ gen_col - !gen_col_r ] in
+                gen_col_r := gen_col;
+                res
+            | Gen_Ori { gen_line = _; gen_col; ori_source; ori_line; ori_col } ->
+                let res =
+                  [ gen_col - !gen_col_r
+                  ; ori_source - !ori_source_r
+                  ; ori_line - !ori_line_r
+                  ; ori_col - !ori_col_r
+                  ]
+                in
+                gen_col_r := gen_col;
+                ori_col_r := ori_col;
+                ori_line_r := ori_line;
+                ori_source_r := ori_source;
+                res
+            | Gen_Ori_Name
+                { gen_line = _; gen_col; ori_source; ori_line; ori_col; ori_name } ->
+                let res =
+                  [ gen_col - !gen_col_r
+                  ; ori_source - !ori_source_r
+                  ; ori_line - !ori_line_r
+                  ; ori_col - !ori_col_r
+                  ; ori_name - !ori_name_r
+                  ]
+                in
+                gen_col_r := gen_col;
+                ori_col_r := ori_col;
+                ori_line_r := ori_line;
+                ori_source_r := ori_source;
+                ori_name_r := ori_name;
+                res
+          in
+          Vlq64.encode_l buf l;
+          loop i (i + 1))
+    in
+    loop (-1) 0;
+    Uninterpreted (Buffer.contents buf)
+
+  let decode (Uninterpreted str) =
+    let total_len = String.length str in
+    let gen_col = ref 0 in
+    let ori_source = ref 0 in
+    let ori_line = ref 1 in
+    let ori_col = ref 0 in
+    let ori_name = ref 0 in
+    let rec readline line pos acc =
+      if pos >= total_len
+      then List.rev acc
+      else
+        let last = try String.index_from str pos ';' with Not_found -> total_len in
+        gen_col := 0;
+        let pos, acc =
+          if pos = last then pos + 1, acc else read_tokens line pos last acc
+        in
+        readline (succ line) pos acc
+    and read_tokens line start stop acc =
+      let last =
+        try min (String.index_from str start ',') stop with Not_found -> stop
+      in
+      let v = Vlq64.decode_l str ~pos:start ~len:(last - start) in
+      match v with
+      | [] -> last + 1, acc
+      | v ->
+          let v =
+            match v with
+            | [ g ] ->
+                gen_col := !gen_col + g;
+                Gen { gen_line = line; gen_col = !gen_col }
+            | [ g; os; ol; oc ] ->
+                gen_col := !gen_col + g;
+                ori_source := !ori_source + os;
+                ori_line := !ori_line + ol;
+                ori_col := !ori_col + oc;
+                Gen_Ori
+                  { gen_line = line
+                  ; gen_col = !gen_col
+                  ; ori_source = !ori_source
+                  ; ori_line = !ori_line
+                  ; ori_col = !ori_col
+                  }
+            | [ g; os; ol; oc; on ] ->
+                gen_col := !gen_col + g;
+                ori_source := !ori_source + os;
+                ori_line := !ori_line + ol;
+                ori_col := !ori_col + oc;
+                ori_name := !ori_name + on;
+                Gen_Ori_Name
+                  { gen_line = line
+                  ; gen_col = !gen_col
+                  ; ori_source = !ori_source
+                  ; ori_line = !ori_line
+                  ; ori_col = !ori_col
+                  ; ori_name = !ori_name
+                  }
+            | _ -> invalid_arg "Source_map.mapping_of_string"
+          in
+          let acc = v :: acc in
+          if last = stop then last + 1, acc else read_tokens line (last + 1) stop acc
+    in
+    (* The binary format encodes lines starting at zero, but
+       [ori_line] and [gen_line] are 1 based. *)
+    readline 1 0 []
+end
 
 type t =
   { version : int
@@ -59,7 +217,7 @@ type t =
   ; sources : string list
   ; sources_content : Source_content.t option list option
   ; names : string list
-  ; mappings : mapping
+  ; mappings : Mappings.t
   }
 
 let empty ~filename =
@@ -69,155 +227,9 @@ let empty ~filename =
   ; sources = []
   ; sources_content = None
   ; names = []
-  ; mappings = []
+  ; mappings = Mappings.empty
   }
 
-let gen_line = function
-  | Gen { gen_line; _ } | Gen_Ori { gen_line; _ } | Gen_Ori_Name { gen_line; _ } ->
-      gen_line
-
-let gen_col = function
-  | Gen { gen_col; _ } | Gen_Ori { gen_col; _ } | Gen_Ori_Name { gen_col; _ } -> gen_col
-
-let string_of_mapping mapping =
-  let a = Array.of_list mapping in
-  let len = Array.length a in
-  Array.stable_sort
-    ~cmp:(fun t1 t2 ->
-      match compare (gen_line t1) (gen_line t2) with
-      | 0 -> compare (gen_col t1) (gen_col t2)
-      | n -> n)
-    a;
-  let buf = Buffer.create 1024 in
-  (* The binary format encodes lines starting at zero, but
-     [ori_line] and [gen_line] are 1 based. *)
-  let gen_line_r = ref 1 in
-  let gen_col_r = ref 0 in
-  let ori_source_r = ref 0 in
-  let ori_line_r = ref 1 in
-  let ori_col_r = ref 0 in
-  let ori_name_r = ref 0 in
-  let rec loop prev i =
-    if i < len
-    then
-      let c = a.(i) in
-      if i + 1 < len && gen_line c = gen_line a.(i + 1) && gen_col c = gen_col a.(i + 1)
-      then (* Only keep one source location per generated location *)
-        loop prev (i + 1)
-      else (
-        if !gen_line_r <> gen_line c
-        then (
-          assert (!gen_line_r < gen_line c);
-          for _i = !gen_line_r to gen_line c - 1 do
-            Buffer.add_char buf ';'
-          done;
-          gen_col_r := 0;
-          gen_line_r := gen_line c)
-        else if i > 0
-        then Buffer.add_char buf ',';
-        let l =
-          match c with
-          | Gen { gen_line = _; gen_col } ->
-              let res = [ gen_col - !gen_col_r ] in
-              gen_col_r := gen_col;
-              res
-          | Gen_Ori { gen_line = _; gen_col; ori_source; ori_line; ori_col } ->
-              let res =
-                [ gen_col - !gen_col_r
-                ; ori_source - !ori_source_r
-                ; ori_line - !ori_line_r
-                ; ori_col - !ori_col_r
-                ]
-              in
-              gen_col_r := gen_col;
-              ori_col_r := ori_col;
-              ori_line_r := ori_line;
-              ori_source_r := ori_source;
-              res
-          | Gen_Ori_Name
-              { gen_line = _; gen_col; ori_source; ori_line; ori_col; ori_name } ->
-              let res =
-                [ gen_col - !gen_col_r
-                ; ori_source - !ori_source_r
-                ; ori_line - !ori_line_r
-                ; ori_col - !ori_col_r
-                ; ori_name - !ori_name_r
-                ]
-              in
-              gen_col_r := gen_col;
-              ori_col_r := ori_col;
-              ori_line_r := ori_line;
-              ori_source_r := ori_source;
-              ori_name_r := ori_name;
-              res
-        in
-        Vlq64.encode_l buf l;
-        loop i (i + 1))
-  in
-  loop (-1) 0;
-  Buffer.contents buf
-
-let mapping_of_string str =
-  let total_len = String.length str in
-  let gen_col = ref 0 in
-  let ori_source = ref 0 in
-  let ori_line = ref 1 in
-  let ori_col = ref 0 in
-  let ori_name = ref 0 in
-  let rec readline line pos acc =
-    if pos >= total_len
-    then List.rev acc
-    else
-      let last = try String.index_from str pos ';' with Not_found -> total_len in
-      gen_col := 0;
-      let pos, acc = if pos = last then pos + 1, acc else read_tokens line pos last acc in
-      readline (succ line) pos acc
-  and read_tokens line start stop acc =
-    let last = try min (String.index_from str start ',') stop with Not_found -> stop in
-    let v = Vlq64.decode_l str ~pos:start ~len:(last - start) in
-    match v with
-    | [] -> last + 1, acc
-    | v ->
-        let v =
-          match v with
-          | [ g ] ->
-              gen_col := !gen_col + g;
-              Gen { gen_line = line; gen_col = !gen_col }
-          | [ g; os; ol; oc ] ->
-              gen_col := !gen_col + g;
-              ori_source := !ori_source + os;
-              ori_line := !ori_line + ol;
-              ori_col := !ori_col + oc;
-              Gen_Ori
-                { gen_line = line
-                ; gen_col = !gen_col
-                ; ori_source = !ori_source
-                ; ori_line = !ori_line
-                ; ori_col = !ori_col
-                }
-          | [ g; os; ol; oc; on ] ->
-              gen_col := !gen_col + g;
-              ori_source := !ori_source + os;
-              ori_line := !ori_line + ol;
-              ori_col := !ori_col + oc;
-              ori_name := !ori_name + on;
-              Gen_Ori_Name
-                { gen_line = line
-                ; gen_col = !gen_col
-                ; ori_source = !ori_source
-                ; ori_line = !ori_line
-                ; ori_col = !ori_col
-                ; ori_name = !ori_name
-                }
-          | _ -> invalid_arg "Source_map.mapping_of_string"
-        in
-        let acc = v :: acc in
-        if last = stop then last + 1, acc else read_tokens line (last + 1) stop acc
-  in
-  (* The binary format encodes lines starting at zero, but
-     [ori_line] and [gen_line] are 1 based. *)
-  readline 1 0 []
-
 let maps ~sources_offset ~names_offset x =
   match x with
   | Gen _ -> x
@@ -230,7 +242,7 @@ let maps ~sources_offset ~names_offset x =
       Gen_Ori_Name { gen_line; gen_col; ori_source; ori_line; ori_col; ori_name }
 
 let filter_map sm ~f =
-  let a = Array.of_list sm.mappings in
+  let a = Array.of_list (Mappings.decode sm.mappings) in
   Array.stable_sort
     ~cmp:(fun t1 t2 ->
       match compare (gen_line t1) (gen_line t2) with
@@ -264,46 +276,48 @@ let filter_map sm ~f =
         loop acc xs
   in
   let mappings = loop [] l in
-  { sm with mappings }
+  { sm with mappings = Mappings.encode mappings }
 
 let merge = function
   | [] -> None
   | _ :: _ as l ->
-      let rec loop acc_rev ~sources_offset ~names_offset l =
+      let rec loop acc_rev mappings_rev ~sources_offset ~names_offset l =
         match l with
-        | [] -> acc_rev
+        | [] -> acc_rev, mappings_rev
         | sm :: rest ->
-            let acc_rev =
-              { acc_rev with
-                sources = List.rev_append sm.sources acc_rev.sources
-              ; names = List.rev_append sm.names acc_rev.names
-              ; sources_content =
-                  (match sm.sources_content, acc_rev.sources_content with
-                  | Some x, Some acc_rev -> Some (List.rev_append x acc_rev)
-                  | None, _ | _, None -> None)
-              ; mappings =
-                  List.rev_append_map
-                    ~f:(maps ~sources_offset ~names_offset)
-                    sm.mappings
-                    acc_rev.mappings
-              }
+            let acc_rev, mappings_rev =
+              ( { acc_rev with
+                  sources = List.rev_append sm.sources acc_rev.sources
+                ; names = List.rev_append sm.names acc_rev.names
+                ; sources_content =
+                    (match sm.sources_content, acc_rev.sources_content with
+                    | Some x, Some acc_rev -> Some (List.rev_append x acc_rev)
+                    | None, _ | _, None -> None)
+                ; mappings = Mappings.empty
+                }
+              , List.rev_append_map
+                  ~f:(maps ~sources_offset ~names_offset)
+                  (Mappings.decode sm.mappings)
+                  mappings_rev )
             in
             loop
               acc_rev
+              mappings_rev
               ~sources_offset:(sources_offset + List.length sm.sources)
               ~names_offset:(names_offset + List.length sm.names)
               rest
       in
-      let acc_rev =
+      let acc_rev, mappings_rev =
         loop
           { (empty ~filename:"") with sources_content = Some [] }
+          []
           ~sources_offset:0
           ~names_offset:0
           l
       in
       Some
         { acc_rev with
-          mappings = List.rev acc_rev.mappings
+          mappings = Mappings.encode (List.rev mappings_rev)
         ; sources = List.rev acc_rev.sources
         ; names = List.rev acc_rev.names
         ; sources_content = Option.map ~f:List.rev acc_rev.sources_content
@@ -331,7 +345,7 @@ let json t =
           | Some s -> rewrite_path s) )
     ; "names", `List (List.map t.names ~f:(fun s -> stringlit s))
     ; "sources", `List (List.map t.sources ~f:(fun s -> stringlit (rewrite_path s)))
-    ; "mappings", stringlit (string_of_mapping t.mappings)
+    ; "mappings", stringlit (Mappings.to_string t.mappings)
     ; ( "sourcesContent"
       , `List
           (match t.sources_content with
@@ -411,8 +425,8 @@ let of_json (json : Yojson.Raw.t) =
       in
       let mappings =
         match string "mappings" rest with
-        | None -> mapping_of_string ""
-        | Some s -> mapping_of_string s
+        | None -> Mappings.empty
+        | Some s -> Mappings.of_string s
       in
       { version = int_of_float (float_of_string version)
       ; file
diff --git a/compiler/lib/source_map.mli b/compiler/lib/source_map.mli
index 6c68720c43..a5d6278329 100644
--- a/compiler/lib/source_map.mli
+++ b/compiler/lib/source_map.mli
@@ -44,7 +44,28 @@ type map =
       ; ori_name : int
       }
 
-type mapping = map list
+module Mappings : sig
+  type t
+
+  val empty : t
+  (** Represents the empty mapping. *)
+
+  val of_string : string -> t
+  (** By default, mappings are left uninterpreted, since many operations can be
+      performed efficiently directly on the encoded form. Therefore this
+      function is mostly a no-op and very cheap. It does not perform any
+      validation of its argument, unlike {!val:decode}. It is guaranteed that
+      {!val:of_string} and {!val:to_string} are inverse functions. *)
+
+  val decode : t -> map list
+  (** Parse the mappings. *)
+
+  val encode : map list -> t
+
+  val to_string : t -> string
+  (** Returns the mappings as a string in the Source map v3 format. This
+      function is mostly a no-op and is very cheap. *)
+end
 
 type t =
   { version : int
@@ -53,17 +74,16 @@ type t =
   ; sources : string list
   ; sources_content : Source_content.t option list option
   ; names : string list
-  ; mappings : mapping
+  ; mappings : Mappings.t
+        (** Left uninterpreted, since most useful operations can be performed efficiently
+          directly on the encoded form, and a full decoding can be costly for big
+          sourcemaps. *)
   }
 
 val filter_map : t -> f:(int -> int option) -> t
 
 val merge : t list -> t option
 
-val mapping_of_string : string -> mapping
-
-val string_of_mapping : mapping -> string
-
 val empty : filename:string -> t
 
 val to_string : t -> string
diff --git a/compiler/tests-compiler/sourcemap.ml b/compiler/tests-compiler/sourcemap.ml
index 791cb94d27..6625996539 100644
--- a/compiler/tests-compiler/sourcemap.ml
+++ b/compiler/tests-compiler/sourcemap.ml
@@ -23,7 +23,8 @@ open Util
 let print_mapping (sm : Source_map.t) =
   let sources = Array.of_list sm.sources in
   let _names = Array.of_list sm.names in
-  List.iter sm.mappings ~f:(fun (m : Source_map.map) ->
+  let mappings = Source_map.Mappings.decode sm.mappings in
+  List.iter mappings ~f:(fun (m : Source_map.map) ->
       match m with
       | Gen_Ori { gen_line; gen_col; ori_line; ori_col; ori_source }
       | Gen_Ori_Name { gen_line; gen_col; ori_line; ori_col; ori_source; ori_name = _ } ->
@@ -110,8 +111,8 @@ function x (a, b) {
 
 let%expect_test _ =
   let map_str = ";;;;EAEE,EAAE,EAAC,CAAE;ECQY,UACC" in
-  let map = Source_map.mapping_of_string map_str in
-  let map_str' = Source_map.string_of_mapping map in
+  let map = Source_map.Mappings.(decode (of_string map_str)) in
+  let map_str' = Source_map.Mappings.(to_string (encode map)) in
   print_endline map_str;
   print_endline map_str';
   [%expect
@@ -128,21 +129,23 @@ let%expect_test _ =
     { (Source_map.empty ~filename:"1.map") with
       names = [ "na"; "nb"; "nc" ]
     ; sources = [ "sa"; "sb" ]
-    ; mappings = [ gen (1, 1) (10, 10) 0; gen (3, 3) (20, 20) 1 ]
+    ; mappings =
+        Source_map.Mappings.encode [ gen (1, 1) (10, 10) 0; gen (3, 3) (20, 20) 1 ]
     }
   in
   let s2 : Source_map.t =
     { (Source_map.empty ~filename:"2.map") with
       names = [ "na2"; "nb2" ]
     ; sources = [ "sa2" ]
-    ; mappings = [ gen (3, 3) (5, 5) 0 ]
+    ; mappings = Source_map.Mappings.encode [ gen (3, 3) (5, 5) 0 ]
     }
   in
   let m = Source_map.merge [ s1; Source_map.filter_map s2 ~f:(fun x -> Some (x + 20)) ] in
   (match m with
   | None -> ()
   | Some sm ->
-      print_endline (Source_map.string_of_mapping sm.mappings);
+      let encoded_mappings = sm.Source_map.mappings in
+      print_endline (Source_map.Mappings.to_string encoded_mappings);
       print_mapping sm);
   [%expect
     {|
diff --git a/compiler/tests-sourcemap/dump_sourcemap.ml b/compiler/tests-sourcemap/dump_sourcemap.ml
index 1d69de2fe9..27dfa136f3 100644
--- a/compiler/tests-sourcemap/dump_sourcemap.ml
+++ b/compiler/tests-sourcemap/dump_sourcemap.ml
@@ -40,7 +40,8 @@ let print_mapping lines (sm : Source_map.t) =
   let lines = Array.of_list lines in
   let sources = Array.of_list sm.sources in
   let _names = Array.of_list sm.names in
-  List.iter sm.mappings ~f:(fun (m : Source_map.map) ->
+  let mappings = Source_map.Mappings.decode sm.mappings in
+  List.iter mappings ~f:(fun (m : Source_map.map) ->
       let file = function
         | -1 -> "null"
         | n -> normalize_path sources.(n)