vsync actually smoothes out the screen by mostly removing screen tearing. The problem is, some setups actually really hate it or do not fully support it so it slows down computers and can ultimate kill games off. Another thing maybe to check would be to see what versions of your graphics drivers and directx are running on compared to the latest. Maybe something there will fix up a small bug you may have? This is if you have this problem overall and not only just KF.
Do you think we can maybe get a small video for us to check out? It's kind of hard what you would explain as "slow".