Framerates aren't bad - but frame pacing is pretty terrible, very noticeable stutter that's consistent enough to look like low framerate.

Dropping internal resolution to 75% (from native 3840 x 2160) improved framerates much more than on windows, you might find you need to as well. The game looks surprisingly good at 75% scaling with TAA high and sharpening.

Another option for resolution scaling:

For a 4K output: gamemoderun gamescope -W 3840 -H 2160 -w 2954 -h 1662 -F fsr -f -- %command%

For a 1440p output: gamemoderun gamescope -W 2560 -H 1440 -w 1968 -h 1107 -F fsr -f -- %command%

For a 1080p output: gamemoderun gamescope -W 1920 -H 1080 -w 1477 -h 831 -F fsr -f -- %command%

Which scales the game down 77% (AMD recommended scaling factor) and uses FSR 1 to upscale to native.

This is worse UI quality than letting he game handle the resolution scaling, but you might find it better for the actual gameplay. And to me the UI looks fine this way too.