This problem applies to pretty much all recording software. You see, when you're recording, the program also has to do a lot of on-the-fly compression. If you're using a more complex codec, you're going to get slower results. Typically it's best to use the lossless codec that comes bundled with the program. FRAPS has its own codec (which is always used for recording), as does CamStudio. If your software doesn't have one, you could try Microsoft Video 1. That doesn't take too much time to encode, so you'll get decent performance. Unfortunately, due to the codec's age, you will only be able to record paletted videos (256-colors max).
If you're using a lossless codec or Microsoft Video 1 and you're still getting poor performance, you may need a better processor.