Blame Fraps.
Even on my sister's computer (which is pretty damn good) it lags in areas.
And I was going to add more stuff like a Steve head and stuff but the greenscreen was acting crappy even though I gave it the best possible circumstances.
Damn you Windows Movie Maker.
1: FRAPS is usually meant for higher end computers that want high definition for more official broadcast. I bought it many years ago, back when Halo PC was still very highly popular. If you have a PC that can run FRAPs better, you should buy it. 30 second clips are less than ideal, especially with the watermark.
2: Windows Movie Maker is terrible. It's made for people who don't know enough to make good videos, and only want to upload this somewhere for their grandchildren to see or something. If you want a quick video, you can use that, but things like greenscreening definitely wouldn't work out on it.