can be different things, there has sadly not been 1 universal cause pinpointed
to some it’s cpu related, to others it’s gpu, or background processes, even something as simple as install+save location or having a antivirus process like active shield/active scan protection running
it’s like a grocery shopping list one might need to go through 
here is a thread going over some stuff High FPS vs. HQ Graphics - the best balance?
i don’t think it covers all, but is like a neat little baseline to try iirc
stuff like CPU trick, disabling active scans/shield for antivirus, ensuring various background processes aren’t interfering, having game install and save on ssd etc i don’t remember if was mentioned