Gamescope
Gamescope creates a virtual desktop for your game. This can be beneficial if your game doesn't play nicely with your desktop, for example if it doesn't catch your mouse, or if it breaks when loosing focus.
You can use Gamescope for downsampling. For example, use gamescope -f -w 3840 -h 2160 -- as command prefix, to downscale from 4k. The same can be achieved in Lutris, by enabling Gamescope, and setting 3840x2160 as resolution.
You can also use Gamescope for upsampling with FSR, by specifying the -U option, for example with gamescope -U --fsr-sharpness 4 -- as command prefix. 4 is an example value, replace it by something between 0 (max) and 20 (min). This also works for native games.
Type gamescope --help in the terminal to see all options.
Setup
Run sudo setcap 'CAP_SYS_NICE=eip' $(which gamescope) to get a smooth gaming experience.[1]
Troubleshooting
You may experience stuttering and lags. Try the following flags:
--rt -r <desired_framerate> --immediate-flips
Replace <desired_framerate> with a number, for example 144.