I'd like to know this too. Right now it's absolutely ridiculous
slow down playback
Yup. makes the entire thing completely useless at this point. One of the many issues with the website in general.

Someone asked this before in the forums, but no mods responded. hopefully one responds here.
Currently from what i've seen there is no way to alter the speed of playback. If u want it slower tha only thing u can do is manually click the next move button

What I do is download the PGN to Xboard and replay on that. You can change the animation speed on Xboard. I agree though Chess.com should add some kind of function for this.

I would like this too, we need the ability to slow down playback when reviewing games. That would help tremendously. Any mods on this?
#metoo
I would appreciate if the playback speed during 'next key move' in game review was adjustable as it's too fast for me to see moves marked as 'good' at present.
I'm trying to improve on the good moves to make them best and stepping through one by one is a little slow.

You can hit the arrow keys both on the website and the ones on your keyboard but other than that I don't think that there is much that can be done, though that would be a useful feature.
Just sayin.

The only thing you can do is record your screen and slow down the speed....
I really think that a speed button would be an added value on this platform, especially for beginners....
silly question: how do I slow down the replay speed (when viewing games, pressing 'play' shows the game moves with too little delay between moves to get the gist of what is going on strategy-wise