Might this be a memory issue? I run it on Xubuntu and noticed it sometimes uses all my 8GB RAM so the PC starts swapping.
Yeah, C:S will gobble RAM like there's no tomorrow, especially if you've got a lot of mods installed.
It loves RAM (and probably RAM speed), but not necessarily in a bad way. Jumping to 16gb (2x8gb) from 8gb (1x8gb) on the laptop I first played it on was a big jump in performance even with a GT740.
Not my experience. Performance is pretty equal for me, always a bit laggy.