Does the problem appear to happen with all vehicles? If yes, then it is something localized. It it happens in only one vehicle, then attach a film to a bug report for that vehicle in the bug report forum.
There are a number of things which can slow a vehicle.
1) Terrain types. Different types of terrain will add drag to the vehicle.
2) Oscillating rudder input, This is normally just noise from the rudder of the stick.
3) Terrain grade. A slight grade is going to slow a vehicle down.
Eliminate all the above, and if it still happens in all vehicles, then attach a film (native AHF file) in the bug report forum.