Encountered this issue yesterday where after i alt tabbed, my game was stuck at 30 fps when tabbed back in. I updated my gpu software (amd radeon software) which have seemed to fix it but today while launching, I alt tabbed and am now stuck with 30 fps. The fps caps in the settings have been set to 100 and restarting my pc does not fix this. When I change the game to windowed mode, it works fine but when i switch back to windowed full screen, i am capped at 30 fps again.
Did you try to set background fps to 100? Do you have vertical sync enabled?
yeah my background fps was set on 100 and vysnc disabled
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Are the power settings of your PC set to maximum performance?
This sounds a lot like a software overlay (probably accidentally) hijacking control over the display driver. Many overlays, especially transparent ones, refresh at a rate of ~30 per second - and they can force that framerate onto the application/game they happen to be projected over.
I’d have a look at tools like Discord, Overwolf, MSI Afterburner, Nahimic, the various programs bundled with video cards that display information… anything like those would be well capable of triggering such an effect. 
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