diff --git a/.travis.yml b/.travis.yml
index 968775e..d1d421b 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,6 +1,7 @@
 language: python
 python:
   - "2.7"
+  - "3.6"
 
 env:
   - PLATFORMIO_PROJECT_DIR=examples/arduino-blink
diff --git a/builder/frameworks/arduino.py b/builder/frameworks/arduino.py
index 7938f9b..e65a4b3 100644
--- a/builder/frameworks/arduino.py
+++ b/builder/frameworks/arduino.py
@@ -116,7 +116,6 @@
         ASFLAGS=["-x", "assembler-with-cpp"],
 
         CCFLAGS=[
-            "-Os",  # optimize for size
             "-Wall",  # show warnings
             "-ffunction-sections",  # place each function in its own section
             "-fdata-sections",
@@ -142,7 +141,6 @@
         RANLIBFLAGS=["-s"],
 
         LINKFLAGS=[
-            "-Os",
             "-Wl,--gc-sections,--relax",
             "-mthumb",
             "-mcpu=%s" % env.BoardConfig().get("build.cpu"),
@@ -166,6 +164,69 @@
             ]
         )
 
+    # Optimization
+    if "TEENSY_OPT_FASTER_LTO" in env['CPPDEFINES']:
+        env.Append(
+            CCFLAGS=["-O2", "-flto", "-fno-fat-lto-objects"],
+            LINKFLAGS=["-O2", "-flto", "-fno-fat-lto-objects", "-fuse-linker-plugin"]
+        )
+    elif "TEENSY_OPT_FAST" in env['CPPDEFINES']:
+        env.Append(
+            CCFLAGS=["-O1"],
+            LINKFLAGS=["-O1"]
+        )
+    elif "TEENSY_OPT_FAST_LTO" in env['CPPDEFINES']:
+        env.Append(
+            CCFLAGS=["-O1", "-flto", "-fno-fat-lto-objects"],
+            LINKFLAGS=["-O1", "-flto", "-fno-fat-lto-objects", "-fuse-linker-plugin"]
+        )
+    elif "TEENSY_OPT_FASTEST" in env['CPPDEFINES']:
+        env.Append(
+            CCFLAGS=["-O3"],
+            LINKFLAGS=["-O3"]
+        )
+    elif "TEENSY_OPT_FASTEST_LTO" in env['CPPDEFINES']:
+        env.Append(
+            CCFLAGS=["-O3", "-flto", "-fno-fat-lto-objects"],
+            LINKFLAGS=["-O3", "-flto", "-fno-fat-lto-objects", "-fuse-linker-plugin"]
+        )
+    elif "TEENSY_OPT_FASTEST_PURE_CODE" in env['CPPDEFINES']:
+        env.Append(
+            CCFLAGS=["-O3", "-mpure-code"],
+            CPPDEFINES=["__PURE_CODE__"],
+            LINKFLAGS=["-O3", "-mpure-code"]
+        )
+    elif "TEENSY_OPT_FASTEST_PURE_CODE_LTO" in env['CPPDEFINES']:
+        env.Append(
+            CCFLAGS=["-O3", "-mpure-code", "-flto", "-fno-fat-lto-objects"],
+            CPPDEFINES=["__PURE_CODE__"],
+            LINKFLAGS=["-O3", "-mpure-code", "-flto", "-fno-fat-lto-objects", "-fuse-linker-plugin"]
+        )
+    elif "TEENSY_OPT_DEBUG" in env['CPPDEFINES']:
+        env.Append(
+            CCFLAGS=["-g", "-Og"],
+            LINKFLAGS=["-g", "-Og"]
+        )
+    elif "TEENSY_OPT_DEBUG_LTO" in env['CPPDEFINES']:
+        env.Append(
+            CCFLAGS=["-g", "-Og", "-flto", "-fno-fat-lto-objects"],
+            LINKFLAGS=["-g", "-Og", "-flto", "-fno-fat-lto-objects", "-fuse-linker-plugin"]
+        )
+    elif "TEENSY_OPT_SMALLEST_CODE" in env['CPPDEFINES']:
+        env.Append(
+            CCFLAGS=["-Os", "--specs=nano.specs"],
+            LINKFLAGS=["-Os", "--specs=nano.specs"]
+        )
+    elif "TEENSY_OPT_SMALLEST_CODE_LTO" in env['CPPDEFINES']:
+        env.Append(
+            CCFLAGS=["-Os", "--specs=nano.specs", "-flto", "-fno-fat-lto-objects"],
+            LINKFLAGS=["-Os", "--specs=nano.specs", "-flto", "-fno-fat-lto-objects", "-fuse-linker-plugin"]
+        )
+    # TEENSY_OPT_FASTER
+    else:
+        env.Append(CCFLAGS=["-O2"], LINKFLAGS=["-O2"])
+
+
 env.Append(
     ASFLAGS=env.get("CCFLAGS", [])[:]
 )
diff --git a/builder/main.py b/builder/main.py
index b46dba3..1b5a026 100644
--- a/builder/main.py
+++ b/builder/main.py
@@ -226,7 +226,7 @@ def _jlink_cmd_script(env, source):
     upload_actions = [env.VerboseAction("$UPLOADCMD", "Uploading $SOURCE")]
 
 # custom upload tool
-elif "UPLOADCMD" in env:
+elif upload_protocol == "custom":
     upload_actions = [env.VerboseAction("$UPLOADCMD", "Uploading $SOURCE")]
 
 else:
diff --git a/platform.json b/platform.json
index 8a3d94d..9a56bb6 100644
--- a/platform.json
+++ b/platform.json
@@ -6,13 +6,13 @@
   "homepage": "http://platformio.org/platforms/teensy",
   "license": "Apache-2.0",
   "engines": {
-    "platformio": "^3.0.0"
+    "platformio": "<5"
   },
   "repository": {
     "type": "git",
     "url": "https://github.com/platformio/platform-teensy.git"
   },
-  "version": "3.7.0",
+  "version": "4.0.0",
   "packageRepositories": [
     "https://dl.bintray.com/platformio/dl-packages/manifest.json",
     "http://dl.platformio.org/packages/manifest.json"
@@ -46,7 +46,7 @@
     "framework-mbed": {
       "type": "framework",
       "optional": true,
-      "version": "~5.51001.1"
+      "version": "~5.51103.190202"
     },
     "tool-teensy": {
       "type": "uploader",