I've recently changed my display from standard 60Hz monitors to 1920x1080 144Hz.
Unfortunately on my Fedora 30 running Cinnamon they're always set to 60Hz frequency.
To get 144Hz mode(or any other custom xorg.conf) you need to:
1) disable Cinnamon's xrandr daemon:
mv /etc/xdg/autostart/cinnamon-settings-daemon-xrandr.desktop /root
2) generate custom xorg.conf config using Nvidia X Server Settings GUI
For me I had to manually set resolution to 1920x1080 and refresh rate to 144Hz for both screens.
Save it as /etc/X11/xorg.conf
Note that by default Fedora does not have that file
3) Reboot and voilà
