I have/had the same things.
Booting was normal after installing 20H2, but the system was very laggy.
I tried a lot: Sfc, DISM, clean boot.
There was maybe a bit of a difference after doing the Sfc (Sfc found some corrupt files) and DISM commands, but not that much.
What helped the most for me, was to set the virtual memory to the recommended paging file size. (I have an SSD HD and 16GB memory)
I had a system managed paging file size before (the paging file size, the system took, was quite big) and that worked well before the update to 20H2.
Windows is showing the recommended paging file size for the system (in my case 2931MB) and I set it fix (by custom size) to 2931 MB.
I set the Initial size to 2931MB and the Maximum size also to 2931 MB.
Now it's much better. It's still not that fast as it was before 20H2, but now I can properly work in my browsers and programs again.
Before this change, there was a delay for almost every click, what's really annoying.
I still think the programs were opening faster before 20H2, but I can live with that, as long as this lags stays away when a program is open.