Running the latest X-Plane for Linux on my MSI GP65 Leopard 9SD works like a charm, and gives at least the same frame rate as under Windows 10. With real weather setting (stormy day), max number of world details, antialiasing and scenery shadows drawn, I still get 70+fps on KDE Neon 20.04.

My laptop:
CPU Intel Core i7-9750H
GPU NVIDIA GeForce GTX 1660 Ti
UHD Graphics 630 (Mobile), Intel
RAM (Memory) 16 GB RAM
Screen Size 15.6 inches (39 centimeters)
Screen Resolution 1920 x 1080 pixels (1080p FHD)
Refresh Rate 144 Hz
I’m running X-plane under KDE Neon 20.04, using drivers from Ubuntu, installed with «ubuntu-drivers autoinstall», giving me version 440.100.
I forced the Nvidia card to max performance using the Nvidia X Server Settings controller, plus «sudo cpufreq-set -r -g performance» to ensure max CPU speed. I was afraid that this would make the fans blow all the time, but they only make noise as long as X-Plane is running. As soon as the simulator is stopped they go silent again. As long as I’m connected to external power I just leave it like this, and as soon as I’m on battery I simply reduce power drainage with «sudo cpufreq-set -r -g powersave».
All in all I’m impressed with what the Ubuntu 20.04 based KDE Neon could do with X-Plane, compared to the fact that I tried it with Windows 10 installed a bit earlier. Roughly the same frame rate, but then W10 had the latest Nvidia Game Ready drivers for the MSI laptop.
I’ve also tested X-Plane under Manjaro Plasma, Manjaro Gnome, Arcolinux and OpenSuse Tumbleweed and they all manage about 20 fps only. On all those distros I used the distro delivered Nvidia drivers and max performance on the GPU and CPU like on KDE Neon. I’ve not been able to find out what Ubuntu has done differently when it comes to the NVidia part, but I can see that the Ubuntu Nvidia contoller is more detailed and allows more tweak than the one supplied with the other distros. However, I only changed to max performance. No other tweaks.
I’ve recently learnt that Laminar Research, the maker of X-Plane is doing their Linux work on Ubuntu.
Update August 18th 2020:
I do get the same frame rates even if I leave Nvidia settings i to «Auto», so obviously the Nvidia controllers are doing their job ok.