Some people find that other applications can interfere with Shotcut. For example antivrius or Grammarly
If you have grammarly, try to remove it. If you have anti-virus programs running, temporarily disable them. If disabling antivirus helps reduce the lag, there are usually settings in the antivirus programs to make exceptions for specific programs. Try to think of any other programs that you may have installed. Temporarily uninstall them as a test to see if they affected Shotcut. Maybe reboot with each test to be sure.