The last time I did any testing was about 6 months ago, but libvpx is terrible at multi-core. x264 scales pretty much linearly with more cores.

I am generally batch encoding thousands of short videos, so I just run the encoder in single thread mode, and create a queue for each core.

IME, VP8 is about half as fast as x264. If you are trying to encode a single video on a 24 core server, then it is about 20 times slower than x264.