its the mouse settings you have in game. most likely the sensitivity is turned way up.
the 40fps you claim your game video is capped at (its likely 50 if you are overseas) has almost nothing to do with your mouse input and is not going to cause the mouse to speed up or slow down.
the only things which control the speed of the mouse are your windows mouse settings, and mouse drivers you have installed and in game mouse settings.
how the mouse acts in windows can be different from in game due to your in game settings. it is also possible to have the mouse act different in different games due to different settings.
if the mouse seems to move more distance when you move quick and not as much when you move slow when you move the same distance on the mousepad then you likely have mouse acceleration turned on. in windows mouse config turn off "enhance precision" in the second tab.