Probably not. If your computer’s performance is too low to run the Shotcut preview smoothly, then the first thing you should do is try to close all your other programs. If it still does not preview smoothly, and you set the priority higher, what would it have higher priority over? Everything else is closed. Maybe 5% of the CPU is being used for background tasks. I predict that Shotcut would still not preview smoothly, but also other things would be unresponsive (like click on the taskbar).
If you try it, we would be interested to know your results.