Go to your Task Manager by pressing CTRL + Shift + ESC.

Navigate to your 'Details' section.

Find the Rainbow Six Siege '.exe'. There should be two; one is the Battleye anti-cheat client (which can be identified by the description), the other is the main '.exe' for the game. Right click the main '.exe' for the game and select "Select Priority". Set this to High.

Source:https://steamproxy.net/sharedfiles/filedetails/?id=739436571