There was a major version upgrade to the UI library in version 20.10, and we had to make significant code changes in 20.11 and 20.01 to adjust. For some people, anything version 20.10 or newer is significantly more laggy, but we do not know why. For others, based on reports, only version 20.11 or 21.01 were significantly more laggy but most recent version 21.02 resolved it. In ALL cases, regardless of version, if you run more than one instance of Shotcut at the same time, anything but the first is slower, and you should avoid that. Make sure you are making very similar comparisons. You might need to remain on an older version that works for you.
Also, make sure you have a clean upgrade. Many people do not. First of all, make sure Shotcut is not already running. It is possible you have a hung instance of shotcut.exe running in the background due to an unclean exit. That could also be the source of extra lag even with a clean upgrade. If you know your way around the Details view of Task Manager, you can check and fix that. Or simply reboot. After that, if you are using the portable, extract it into a new folder. If you are using the installer, let it remove the old files or give it a new destination folder.
If you are double-clicking a .mlt file to open Shotcut with that project, it might not be opening the version you want. Shotcut installer does not register .mlt and only you can do that using Explorer > Open With… and then it simply remembers the exe that was chosen and not aware of new installs unless it is an in-place upgrade.