I'm noticing the same. It's very laggy using quicklook.It's not indexing anymore. I might try all those steps later but right now I have to get back to work. I am sceptical though clearing caches will help with quick look. With small files it's still fast. However try using it with let's say a photo and it is slide show. It's nuisance, but in Catalina it was buttery smooth.
Just noticed this as well. That's an insane UI decision. Fingers crossed for the Mail opening by itself bug that was in infuriating one!Continued.. Wow! Mail notifications lost instant delete button. Now you have to click 'Actions' and then 'Delete'. Usability is getting better and better. I used it a lot, another great addition.
EDIT: Good news is, Mail no longer seems to open by itself. However I'd give it yet more time.