Detailed PC specs are necessary before we can tell you the most likely cause.
However we can speculate that you either haven't patched your game fully, your drivers aren't updated, your system doesn't meet the game's requirements fully, or you're suffering from heat throttling or faulty hardware.
However, to truly pinpoint the problem we need your detailed system info.
Appy All The Patches From 1 To 4.. And You Will Get a major Boost!
I have an Intel C2D E4500 @ 2.6 Ghz And ATI 6750 1GB..
At First Black Ops Lagged A Lot.. But After i applied the patches upto 4, it runs as smooth as butter on high texture quality at 1280 x 800 Resolution!
Hey guys um.. sory for not answering for a long time but i tried lowering the graphics itself and it went fast.. well if i dont shoot anything that involves wooden stuff that makes smokes and stuff.. but thanks for helping...