Hi Christian,
The result of your testing in Safe Mode suggests there is a bad or duplicate font on your Mac. There is also a known issue with non-Microsoft fonts and PowerPoint version 16.9 update and later. You can't do anything about non-Microsoft fonts showing up until there's an update, but the typing speed issue seems to be related to a bad or duplicate font.
Follow instructions here - except select ALL your fonts (Command-A) before running the validate fonts command so you don't spend all day validating them one at a time.
Font Book for Mac: Validate fonts in Font Book - Apple Support