This provides a pair of actions, Custom: Transport: Double Playrate and Custom: Transport: Halve Playrate, which are relatively self-explanatory. The double playrate action is a lot like the fast-forward action but without altering playback state.
REAPER only provides actions to increase and decrease by semitone, so this custom action chains 12 of those to create an octave. It also laces all but the final action with the OSARA mute next message action so your screen reader doesn't get spammed with every intermediate rate.
Using these allows you to quickly operate at absurd playrates, I've observed REAPER chugging along at 64x with pure audio. Note that involving any FX at all is likely to break down, and if you reset the playrate to 1 during playback with FX applied a massively glitched buffer underrun results until it catches up to itself.
FX do not appear to actually run with their time scaled by playrate, so e.g. an EQ will quickly fall out of sync at higher rates because the frequencies have changed.
The keybinds I use for these (on Windows) are control+alt+shift+dash and control+alt+shift+equals, similar to the actions which adjust playrate by semitone.