Thanks for the answer!

I don't think xvid will solve the problem, cvWriteFrame is too slow as soon as the videofile is 2 MB or so. I use an usb cam but the quality isn't a big problem right now. The problem is that more and more fps are dropped as the videofile gets bigger. The only solution I can think of is to keep all frames in RAM until the recording is finished, and then write it to the file. But it is encoded to h264 with cvWriteFrame so if i save them in RAM, it is in the raw-format which taks a lot of memory even for, say 256*192 pixels.