diff --git a/tests/wpt/README.md b/tests/wpt/README.md
index 9d7acda598f..05c746c4e0a 100644
--- a/tests/wpt/README.md
+++ b/tests/wpt/README.md
@@ -43,6 +43,15 @@ to run all the DOM tests.
 There are also a large number of command line options accepted by the
 test harness; these are documented by running with `--help`.
 
+Running all tests
+------------------------------
+
+Running all the WPT tests with debug mode results in a lot of timeout.
+If one wants to run all the tests,
+build with `mach build -r`
+and
+test with `mach test-wpt --release`
+
 Running the tests without mach
 ------------------------------