This is an age-old PC problem.

1) Turn off v-sync in your display properties
2) If this doesn't help then look for an option like "render frames ahead" and make sure that is set to 0
3) Turn off Anti-Aliasing in your graphics card options and failing that, in the game

and everyone remember the above to combat the dreaded "cursor lag".

edit: I remember that vegetation detail can have an effect on this too.