diff --git a/pageserver/src/l0_flush.rs b/pageserver/src/l0_flush.rs
index 7fe8fedc6394..8945e5accdbf 100644
--- a/pageserver/src/l0_flush.rs
+++ b/pageserver/src/l0_flush.rs
@@ -2,13 +2,23 @@ use std::{num::NonZeroUsize, sync::Arc};
 
 use crate::tenant::ephemeral_file;
 
-#[derive(Default, Debug, PartialEq, Eq, Clone, serde::Deserialize)]
+#[derive(Debug, PartialEq, Eq, Clone, serde::Deserialize)]
 #[serde(tag = "mode", rename_all = "kebab-case", deny_unknown_fields)]
 pub enum L0FlushConfig {
-    #[default]
     PageCached,
     #[serde(rename_all = "snake_case")]
-    Direct { max_concurrency: NonZeroUsize },
+    Direct {
+        max_concurrency: NonZeroUsize,
+    },
+}
+
+impl Default for L0FlushConfig {
+    fn default() -> Self {
+        Self::Direct {
+            // TODO: using num_cpus results in different peak memory usage on different instance types.
+            max_concurrency: NonZeroUsize::new(usize::max(1, num_cpus::get())).unwrap(),
+        }
+    }
 }
 
 #[derive(Clone)]