Are there any hardware MP3 players that allow you to change the playback speed?

One of these is reasonably cheap and readily available and can change the speed of podcast playback.

https://www.bestbuy.com/site/sandisk-clip-jam-8gb-mp3-player...

While it is not a "hardware player" the "Voice Audiobook Player" on F-droid (for Android phones) does have an adjustable playback speed:

https://f-droid.org/en/packages/de.ph1b.audiobook/

Olympus digital voice recorders (check online manuals before you buy), I have an old one, DM-5 that allows you to change the playback of MP3s from 50% to 600%! only minus with this is friggin slow startup if you add a lot (over 1000) MP3s plus bit PITA to use under linux.

Rockbox, which is replacement firmware that can run on many hardware players, can do this (and basically anything else).

https://www.rockbox.org/

I usually don't do this, actually. Especially if I'm watching testimony in legal cases, something I do often.