I had terrible problems with impossibly laggy performance, despite having a decent setup (i7; 16GB RAM; plenty of disk space; W10).
Tried most of the things recommended in the thread here without benefit.
The one thing that did work was installing an earlier version (20.07.27) which now works fine.
Apparently, some of the later updates clashed with the later OS and just wouldn’t behave.
Info on this avialble here: Multiple Installations of Shotcut - Windows 10
Anyway, it works fine for me now—if all else fails, hope it does for you, too! 