Boot up and open either Task Manager or Resource Monitor. Both are utilities built into Windows.
Observe your system for a while and explore the utility to learn how to navigate around and read the information presented.
Then slide the window to one and leave it open. Game as usual. Continue watching the utility's output.
Look for some parameter change that occurs with the mouse movements you make and the corresponding slow downs.
Watch background apps (backups, AV, utilities, etc.). Could be that one of them is involved. Some conflict.