I have had a heck of a time diagnosing this and the fact that it happens in multiple games leaves me to thinking it's something about the way my 5700XT is set up. It's a few months old but all the config stuff is basically out-of-the-box. I haven't monkeyed with the Radeon software.
In the following games I get very low FPS but if I tab out (or just hit the Windows key) the FPS instantly jumps up to 60, which is my monitor and Windows refresh rate, and proves it's not a hardware capacity issue. Occasionally relaunching the game addresses the problem but it's too unreliable to be useful (around 25% of the time). On the other hand, tabbing out and back in when things work has a chance to break the FPS, too.
- Sea of Thieves (Steam) - hard capped at exactly 40 FPS until the game doesn't have focus
- Deep Rock Galactic (Steam) - struggles in the mid-30s until the game doesn't have focus
- Satisfactory (Epic Games Store) - FPS in the high 20s until I enabled DX12 (which is experimental and hasn't worked on subsequent loads - might be a coincidence)
Clearly there is something going on where the drivers are throttling games in windowed full screen when they have focus but they have no trouble rendering beautifully when they aren't the top app. FYI I am reporting the in-game FPS counters which means no 3rd party software is misreporting a refresh rate on portions of Windows that show up when I hit Start or alt-tab to something else. The difference is also abundantly clear just watching them run in the 'background,' so the FPS numbers are backed up by other evidence.
I have seen similar issues posted here and elsewhere from 2016 on but never a solution. At this point I see no reason not to go back to nVidia the next time I upgrade. I think AMD has done some great things for consumers and I want to support them but not if I can't play my games.
Specs (since I know someone will ask):
Windows 10
Ryzen 7 3800X
Ryzen 5700XT
32GB RAM
