STEP 1.
Start your video game.
STEP 2.
Ctrl +alt + delete and task manager will appear.
STEP 3.
Click the processes tab.
STEP 4.
If its a game look for the main application (for example hl2.exe) and right click it.
STEP 5.
Click 'set priority'.
STEP 6.
Click 'high', or if it's severe click 'realtime'.