Well they published their source code.

It’s not as permissively licensed as usual open source projects, but I would argue that in the true sense of the phrase, this falls under “open source”.

I am running Manjaro KDE on two general purpose PCs, both with Nvidia graphics, both without problems.

On my living room gaming setup (also Nvidia), I am running Bazzite, which also runs very nicely.

I think I have some bad news about your ex gf 💀

Are you using multiple screens and have something open on another screen that periodically rerenders? I have had a similar problem when having Ksysguard open on a second screen - every time the graphs rendered a new time step, my game lagged. Only happened on Wayland. My solution was to run the game inside of gamescope or to close the offending application.