Everything in my surroundings moves smoothly but if I move the mouse to look around it looks really laggy as if the frame rate was low (but it isn't!). What's up with that? Is it my mouse? Is it the game? Why does it do this? :P

Extra info if needed:
I have...
-- all graphical settings maxed out except for 'volumetric smoke'.
-- the latest nvidia drivers installed.
-- Steelseries Rival gaming mouse with latest Steelseries software installed.
-- the Origin/Warchest version of the game.
-- tried running the game in compatibility mode; XP service pack 2, 3.

Specs & OS:
Z97-A
i5-4690K
GTX 970
Samsung 850 EVO
16GB RAM
CX600M 600W
Windows 7 64-bit Home SE

Thanks in advance!