Something truly weird is going on here. Let's say I have a variable which is not defined. I add the definition for it. The undefined red underline doesn't disappear. If I cut and reinsert the row , it immediately gets cleared. Same happens with unused variables, undefined functions etc. The IDE seems snappy otherwise.
Win10 + WSL1 here. I/O is slow, no question. But 2020.1 worked fine.