Safari does not support vp8 or vp9 when playing youtube, and youtube serves h264 instead. h264 is less efficient in terms of compression ratio (more to download for the same quality), but h264 is decoded in hardware on OSX, and VP8 or VP9 isn't, which explains what you see.
This is why, for example, Safari does not have 4k video on Youtube, while being perfectly capable of playing 4k videos in general.
Depending on the machine, VP9 can be decoded in hardware on Firefox on Windows, but chip support is limited.
All that said, we're working on our video playback performance as we speak, especially on OSX (because it was so bad a few release back), but also in general.
This is why, for example, Safari does not have 4k video on Youtube, while being perfectly capable of playing 4k videos in general.
Depending on the machine, VP9 can be decoded in hardware on Firefox on Windows, but chip support is limited.
All that said, we're working on our video playback performance as we speak, especially on OSX (because it was so bad a few release back), but also in general.