Ubuntu 20.04 - how to fix sluggish 4K monitor

HDMI cable

Cable matters. By changing the HDMI cable, the refresh rate changed from 30 to 60.

Refresh rate

Check the refresh rate. There are many reports the refresh rate stuck to 30 hz.

$ xrandr
Screen 0: minimum 1 x 1, current 3840 x 2160, maximum 16384 x 16384
Virtual1 connected primary 3840x2160+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
   3840x2160     60.00*+   <--- Make sure 60 hZ is there
   2560x1600     59.99  
   1920x1440     60.00  
   1856x1392     60.00  
   1792x1344     60.00  
   1920x1200     59.88  
   1600x1200     60.00  
   1680x1050     59.95  
   1400x1050     59.98  
   1280x1024     60.02  
   1440x900      59.89  
   1280x960      60.00  
   1360x768      60.02  
   1280x800      59.81  
   1152x864      75.00  
   1280x768      59.87  
   1024x768      60.00  
   800x600       60.32  
   640x480       59.94  

Fractional scaling

Avoid fractional scaling.

• Big performance degradation in X11 + 4K resolution + fractional scaling in second monitor compared to Wayland

This is expected. Fractional scaling in Xorg sessions works by rendering at a higher resolution than your screen(s). So that increases the likelihood of stuttering compared to integer scaling.

That specific issue will never be fixed because it's a required trade-off to get fractional scaling working in Xorg. HOWEVER, general performance will increase a lot in future versions like Ubuntu 20.10 so that hopefully will be enough to offset and eliminate all the problems you see.

Font size

With high resolution 3840x2160, the default font can be too small. See How do I increase the font size in Ubuntu 18.04 LTS for a HDPI / 4K display?. I used Gnome Tweak scaling factor.

Apparently it also enable Setting > Universal access > Large text.