I had this problem too and fixed it by opening the MacBook, cleaning the fans and replacing the CPU thermal paste.
The heat wasn’t leaving as expected before, which caused the overheat up to 95 degrees! And that caused the CPU throttling to kick in and make the laptop very slow
All good now, after the cleaning. You’ll need special screw drivers, thermal paste, a pressured air can (all easy to order online), patience and one of the videos on YouTube that explains how to do it step by step. Make sure you search for your model.
It’ll take you two hours but it’s worth it, if you’re not up to taking it in for cleaning.
Good luck!