Game lagging when changing the speed of a locomotive

Hi everyone, I have this weird thing that when I change the speed of a locomotive in Driver, the game just drops a lot of frames. I'm going to like 60 to 20, then 30, 20, then after the engine reaches the speed, the game starts running at 60fps again. Can anybody give an explanation for this? thanks

Does the same thing happen in different circumstances?
e.g.
- same loco - different performance settings
- same loco, different location
- same loco different route
- same route, different loco
- change speed using DCC vs cab mode vs AI command
etc