Yes sorry, the above certainly uses plasma tools, and I didnt mention it.
(though … making it was the first time I used those plasma tools … it was first being made for xrandr and can pretty quickly be adopted for//instructive of that method as well)
Hm.
Without access to an XFCE system its a bit difficult for me to double check.
But a few things come to mind.
First - compositor. (check turning it off … xfwm compositor is … weird)
Second (and mainly) - generally systems are set at 60 by default. (insert my long story here)
[check running glxgears vs vblank_mode=0 glxgears]
… I guess at this point I am wondering if you need pointing to the similar xrandr commands … or pointing towards the wiki for ‘manually adding and using display resolutions’