- Opera 10.60: ~110% (i.e. hogging more than one core)
- Nightly Firefox build "Minefield": ~110%
- Safari v4: ~20%
For comparison, the "old" Flash player was taking ~50% CPU.
It seems that both Opera and Firefox are using the VP8 codec for video, which is currently not hardware accelerated (at least on OS X, due to Apple not standardising some API for GPU use). Safari is using the H.264 codec which clearly performs much better on this machine, but which is patent and licence encumbered.
So at this point, VP8/WebM is completely unusable for watching video on slowish machines like mine (a 3 year old Intel Macbook). Here's hoping that VP8 can take advantage of better hardware rendering soon...
* Minor addendum: Looking at this analysis of VP8, perhaps the encoder/decoder implementations are just immature at the moment and need to be optimised in a few areas. However, there are also some mentions of disturbing (in terms of possible future patent issues) similarities between the VP8 and H.264 designs.