I've been using Spacedesk on Windows with two old Android tablets. They're Samsung Note 2014s, super cheap, old enough that YouTube has stopped working, but they're 10" at 2560x1600 with good color.

With good wireless, the setup is reasonably performant, fast enough to move the mouse and type fluently. Certainly slower than USB monitors, but ideal to put some reading material on one screen (PDF or a browser window), and a text editor on the other while still using the main screen.

I also use them in portrait orientation 10:16, which is nice. Cheapest and most portable triptych setup there is, I guess.