Just to get a baseline on what you've done, when you said:
Did you do a clean install onto a new or existing drive, or did you just reinstall in place? (Something far less representative)
I've had some strangeness over the years, and if I really want to get down to it, I have to do a clean reinstall of windows. If it was me in your situation (Everything is working fine here) I would get an SSD and reinstall windows and PP and then I would have something real to say if it doesn't work.
However, in the last two updates, I cleared my cache manually, and the first times I ran the new update, my system was acting strangly and slow... but then after I started it a few time and let it sit. Then everything was fine in both instance, where it looked a bit dire earlier and PP appears snapier in each new version, so I think the development team is doing a good job in working on performance. (However did we realy need a "Quick Export" button?
Also you said:
Creating a new project and imported the old project
Did you just try it with just a new project (no importing). Drop in some basic HD clip, h.264 or prores type thing and give it a try.
When I said I reinstalled Windows, the whole computer was wiped. My C Drive (where Windows + Premiere etc is installed) is a 1T SSD. I have another 1T SSD for cache files.
And I have imported old projects with a variety of formats including h.264 and still have the same problems.
One thing of note...
A specific problem for Premiere 2020 was EXTREMELY sluggish controls. So much so editing was literally impossible. But I noticed when this was happening only 3 of my 28 logical processors were being used - and two of them maxing out. So I changed some settings on my nvidia control panel (including switching thread optimisation on) which seemed to improve this problem: all logical processors are now being used, and controls have improved - not perfect, but useable. However problem of dropping frames remains in both 2020 and 2019 versions, which is making it very difficult to edit fast with precision.
Back in the day, I used to love using Premiere because it would ingest and play pretty much whatever you gave it. Now it doesn't seem to play ANYTHING fluently. If I could transfer this project to CS6 I would...
PLEASE let me know if you have any ideas on how to fix this.