Framerate drop when packaged game out of focus

I know it’s an old topic but I just had the same issue when testing Lyra project, and found answer in another post.
Turn out it was an issue with fps set to unlimited so the focused Windows was taking almost all the performance letting very little for the other out of focus instance:

https://forums.unrealengine.com/t/packaged-game-drops-fps-when-not-focused/580679/4