No need for an extension -- I'm paranoid and rather not install anything unless absolutely necessary -- when a simple one-liner bookmarklet will do:

  javascript:(() => { document.querySelector("video").playbackRate = 16; })();

I make a few copies for different rates (1.75x, 2.5x, etc.); this is the one I use to fast-forward ads whenever my ad-blocker fails.

Love this extension. I watch almost everything at 2.5x now.