TLDR; It doesn’t work so nicely
I recently purchased a 4k display to go along with my Thinkpad T480 with GPU. For the first few days I used in 1440p as I didn’t have the USB C to HDMI cable for a 60hz (Thinkpad’s HDMI was not 2.0) . Then I bought one.
Ubuntu didn’t support fractional scaling on LTS release before 20.04. Since I am on Ubuntu 20.04 I thought this would work fine. I connected the cable and switched to 4k 60hz. That display was too small for my liking. I immediately switched to 125% scaling which is supported out of box on Ubuntu 20.04. I immediately noticed a serious lag. I checked what fps i am getting and got to know that it is around 24-32. So i switched back to 100% scaling and checked again. I am getting 59 fps. Then i checked all fractional scaling and saw there is a performance dip. Only scaling I didn’t get dip in performance was at 100% and 200%. I searched and saw that there is a performance hit for fractional scaling on Ubuntu. Some people have commented to use wayland session. But due to the nvidia drivers I cannot switch to wayland session.
I got annoyed and started testing other DE’s like cinnnamon, kde, deepin. I was more annoyed by other DE’s with things unrelated to fractional scaling. Uninstalled everything and came back to gnome.
Now I had some options. To use it too small at 100%. Or increase font scaling at 1.25 or use 1440p or use at 200% scaling. I tried font scaling 1.25, it doesn’t look proportional and i really didn’t want to use 1440p. 200% is too large. So ultimately I started using 4k display at 60hz with 100% scaling by turning on font scaling for 1.25.
I might ditch Ubuntu and go to windows with proper fractional scaling and use WSL2