diff --git a/python/servo/build_commands.py b/python/servo/build_commands.py
index ca9075a48f00..ee96d5be69a5 100644
--- a/python/servo/build_commands.py
+++ b/python/servo/build_commands.py
@@ -96,6 +96,9 @@ def build(self, target=None, release=False, jobs=None, android=None,
         opts = params or []
         features = []
 
+        if not release:
+            print("Warning: This is an un-optimized debug build. For performance testing, use ./mach build --release to enable compiler optimizations.")
+
         if release:
             opts += ["--release"]
         if target: