Do you mean running down a ramp, or in general running on a plane surface too?
The "running down a ramp with pulled handbrake" is very annoying indeed, and I have no clue why this movement code was "really necessary" ... developers said, if they did not do that, people would permanently suffer falling damage while riding.
But extreme lag while running on plane surfaces would be rather surprising. I could only imagine that harddisk access (loading NPCs with their attributes) pauses the movement, as this happens extremely for me.