Nvidia!
That's the problem. Not that it's a lot better with other graphics chips, except when you are lucky.
You only need the correct graphics driver, which you can download from the Nvidia site.
Much easier said than done - it has taken me days of googling in the past.
But twice recently I have been gobsmacked by KDE. Install KDE5, and a pop-up box recognises the situation and offers to find and install for you the correct driver. And that works perfectly.
You can probably then deinstall KDE and go back to Xubuntu.