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.
CPU: AMD Ryzen 3600 @3.6Ghz
GPU: Asus TUF AMD Radeon 5600 XT
RAM: 16GB DDR4 @ 3200Mhz
SSD: Samsung M.2 SSD