diff --git a/build.ps1 b/build.ps1
index c0de889..cb16ce0 100644
--- a/build.ps1
+++ b/build.ps1
@@ -169,6 +169,11 @@ function Invoke-Swift([string[]]$SwiftArgs, [hashtable]$Sql) {
     # carries the prefix — it is global, so it also downgrades any FUTURE duplicate to a
     # warning. Read the warnings.
     $full = @($SwiftArgs) + @(
+        # Index-while-building costs real per-file work (`-index-store-path` +
+        # `-index-system-modules` on every frontend job) and nothing consumes the store from a
+        # command-line build — it exists for editors. Off here; CI leaves it at the default,
+        # where a cold index nothing reads costs nothing either way.
+        '--disable-index-store',
         '-Xcc', "-I$($Sql.Include)",
         # `/LIBPATH:`, NOT `-LIBPATH:`. A leading `-L` is claimed by the driver's GNU-style
         # library-path flag, so `-LIBPATH:C:\…\lib` is read as `-L` + `IBPATH:C:\…\lib` and