As for rewinds, you can click on the progress bar to jump to a time you want. Also, the upcoming version of the player supports fast-forward/rewind/jump keyboard shortcuts: https://github.com/asciinema/asciinema-player/blob/next/src/...
As for the playback speed you can set speed, theme, font size etc (embedding docs say this - https://asciinema.org/docs/embedding), and you can append these options to any asciicast URL too, like this: https://asciinema.org/a/21743?speed=5&theme=solarized-dark
Looks like these are not easily discoverable from the player UI, so noting this down. Thanks for bringing this up!