diff --git a/t/perf/README b/t/perf/README
index bd649afa97..fb9127a66f 100644
--- a/t/perf/README
+++ b/t/perf/README
@@ -28,6 +28,8 @@ the tests on the current git repository.
     7810.3: grep --cached, cheap regex       3.07(3.02+0.25)
     7810.4: grep --cached, expensive regex   9.39(30.57+0.24)
 
+Output format is in seconds "Elapsed(User + System)"
+
 You can compare multiple repositories and even git revisions with the
 'run' script: