What output video format for Kazam requires less cpu power?
VP8 and H.264 should essentially be the same demand due to them being similar. 

RAW is well, RAW, each specific pixel data, with no methods of reducing file space, this can cause very large file sizes, but is in theory the least intensive as long as your hard drive can record fast enough. 

HUFFYUV is basically the same as RAW, however it uses some methods to reduce file size. Depending on the footage it can be about half the size, or even up to 1/10th the size of RAW. 

Lossless JPEG, I believe is similar to HUFFYUV, however I don't personally use either of these lossless codecs. 

I don't know if Kazam allows you to use custom codecs, but if it does, I highly recommend using the Lagarith Lossless Codec. It's what I personally use, and it has some basic GPU support, very nice file sizes without any loss in quality, and doesn't require any tinkering which some lossless codecs do.