This has been going on for about a year now, or at least since I got my new system completed. The in-game counter will stay at a solid 60 FPS all the time, however, if I go full screen, the FPS drops to 5, and maximized window feels like 60 most of the time, but it repeatedly drops to 15 - 30.
This seems like an issue with Roblox not realizing it’s in focus, and thus, uses less resources, resulting in much lower frame rates. Roblox is also the only application that does this, so I’m confident it’s not my PC or my GPU settings.
I’ve had this issue before, and I’ve had others talk about it. Try pressing Alt + Enter after entering full screen with F11; this may solve the issue (and it should also cause your GPU software to recognize the application as full screen.)
I am having the same problem. My GPU is Nvidia 1060 (6GB). Its not consistent, sometimes its rough but other times I have to reboot my computer to make it smooth.