Why does FRAPS lag my game?

I have a decent computer. I've bought FRAPS and everything, I've recorded 3+ hours of footage with no lag, but when I record Minecraft, it drops the framerate so much. I go from like 300+ FPS to like 20 and below. I doubt it's my computer. I've set Minecraft's priority to realtime, defragged my drive. Everything I could think of.

To help the lag you should put a fps limit, and when FRAPS is turned on to record it takes away 10-20 fps.