Copter-3.4 includes support for Terrain Following in Auto mode.But it occurs to FailSafe when the Copter flights at 3m.
You need either a rangefinder or a GCS that supports terrain following.
In auto mode, the speed of Copter flights 1 m/s, and it can have a normal flight. howerver at the speed of 3 m/s, the terrain following failure, using the lidar.
I think speed shouldn’t make any difference in regards to terrain following. Anyway, if you share a log there’s a chance someone looks at it and is able to help you.