Hey there,
Is this a laptop? If so it is likely you are using both an integrated and dedicated GPU. The integrated GPU is the one the system will use by default for everyday tasks. It is supposed to switch from that integrated to the dedicated mobile GPU when you launch a graphic intense process like a video game.
Issues of low FPS or a sustained FPS drop after alt tabbing could be due to an issue with this switching process. (IE when you tab out, the system switches back to the integrated GPU but doesn’t switch back to the integrated when you tab back in.)
Ensuring you have all driver updates for both GPUs can help.
Now that was assuming this was a laptop. If the above doesnt solve the issue or doesn’t apply here because you are on a desktop, then the community needs more information on your setup to try and offer usable troubleshooting.