I have the exact same issue on my 2020 MBP (4 Thunderbolt + 1TB SSD version) but I did encounter it as well on a 2020 Macbook Air (CTO) with the i5 & 16GB RAM. In both cases I had the project and files on an external Samsung T5 SSD. On the MBP I also moved the files on the internal storage just to see if it solves the issue, but it did not. I find it unacceptable that these brand new laptops have these issues considering I was able to flawlessly edit on my old 2015 i7 MBP (integrated graphics).
Strange thing is that on the MBA, I tried to export the files via Compressor (as I usually do) but I could not because it always gave me an error and I had to do the export via FCP X. The solution was to cleanup both the FCP and Compressor installation and install them again.