diff --git a/libraries/AP_L1_Control/AP_L1_Control.cpp b/libraries/AP_L1_Control/AP_L1_Control.cpp
index 6dc3557f22..6b7429869a 100644
--- a/libraries/AP_L1_Control/AP_L1_Control.cpp
+++ b/libraries/AP_L1_Control/AP_L1_Control.cpp
@@ -73,6 +73,7 @@ float AP_L1_Control::crosstrack_error(void)
 }
 
 // update L1 control for waypoint navigation
+// this function costs about 3.5 milliseconds on a AVR2560
 void AP_L1_Control::update_waypoint(const struct Location &prev_WP, const struct Location &next_WP)
 {