Apparently, there was a certain code implemented that purposely will slow down your gaming experience in MW3's multiplayer. This issue is called lag compensation, and it makes MW3 very slow and hard to play.
Now, let's explain this whole MW2 > MW3. A case study: Play a few minutes of MW2 MP and a few minutes of MW3 MP. Notice the difference? Yes, MW2 is a well-seasoned gameplay, whilst it's younger sibling is not.
Wait until the developers fix the game. Right now, it has balance issues and terrible lag issues. (Talking about the multiplayer) If you want a fun, fast-paced, smooth gaming experience stick with MW2. If you're playing Black Ops, dump that garbage (pronounced GAR-BAJ, man), and pick up MW2.
@CandiBunni: Creepy voice guy sounds off: http://www.youtube.com/watch?v=wJOpFCCnv0E
MW2 feels much smoother. Among several other things, MW3 is a lagfest and was not balanced correctly coming off of MW2. MW3 is a lot slower and more punishing than MW2. MW2 is much faster and less punishing than MW3.