diff --git a/.changeset/rsbuild-cheaper-compilation-scans.md b/.changeset/rsbuild-cheaper-compilation-scans.md
new file mode 100644
index 00000000000..9a9d015f68f
--- /dev/null
+++ b/.changeset/rsbuild-cheaper-compilation-scans.md
@@ -0,0 +1,5 @@
+---
+'@tanstack/start-plugin-core': patch
+---
+
+Skip server function metadata validation for Rsbuild modules that have no server function metadata.
diff --git a/packages/start-plugin-core/src/rsbuild/start-compiler-host.ts b/packages/start-plugin-core/src/rsbuild/start-compiler-host.ts
index 3cd1725fa09..cae59ce7c39 100644
--- a/packages/start-plugin-core/src/rsbuild/start-compiler-host.ts
+++ b/packages/start-plugin-core/src/rsbuild/start-compiler-host.ts
@@ -82,9 +82,13 @@ function resolveMetadataLoader(): string {
 function readServerFnBuildInfo(
   module: Rspack.Module,
 ): Record<string, ServerFn> | null {
-  const result = serverFnBuildInfoSchema.safeParse(
-    module.buildInfo[SERVER_FN_BUILD_INFO_FIELD],
-  )
+  const metadata = module.buildInfo[SERVER_FN_BUILD_INFO_FIELD]
+  // Most modules have no metadata, and zod's failure path dominates replay.
+  if (metadata === undefined) {
+    return null
+  }
+
+  const result = serverFnBuildInfoSchema.safeParse(metadata)
   if (!result.success) {
     return null
   }