To disable Defender from checking in the background while
you're using the computer,
you need to disable "Real-time protection", like this :

- Run *Settings > Privacy & security > Windows Security*
- Click "Virus & threat protection"
- In the *Virus & threat protection* window, under
"Virus & threat protection settings" click "Manage settings"
- In the *Virus & threat protection settings* window, set
"Real-time protection" and "Dev Drive protection" to Off.

The effect is immediate, no reboot required.
While this setting is in effect, Defender won't do any checks,
although it's still possible to manually start disk scans.

You should undo this when you're not playing the game.