I was having similar issues. A combination of two solutions posted by other people in another thread solved it for me. Game is freezing when I alt tab - #26 by Pazzik-daggerspine is my thank you post for their help.
Short version of what fixed it for me: disable Hardware Acceleration in other running applications (Discord, Battle.net app, Web Browser, etc) and set Max Background FPS to exactly match your chosen Max Foreground FPS setting.