This is the process that seems to work, and it is best to do it step by step, precisely as detailed below:
That includes cleaning of the registry which is important. However, what you described seems quite bizarre, but the 30% CPU doesn’t tell you anything useful about what’s really going on (other than you know Shotcut isn’t gobbling that all up) so I’d check windows updates, let them all run and then reboot before doing the shotcut uninstall/reinstall process above.
Two other things to try if the above doesn’t work; go for a full uninstall and then try a different version (or even the executable, instead of the portable version), or install on a new user (windows) profile.