Issues with ALT-TABBING in game with W10 (Not sure if other versions does the same).

If I ALT-TAB numerous amount of times my FPS caps betweent the values of 60-90, it's highly annoying since I don't use a dual monitor so when checking things like stream chat / changing music / checking skype/discord, eventaully my FPS caps resulting in me having to restart FGunZ.. I seen streamers like Nerval / Barbitton able to ALT-TAB numerous amount of times and this not happen to them, is there a setting I need enable or something I need to update?