Mi Pad 4 8" very laggy ui, changed renderer, disabled miui optimisations, same result. E.g app drawer on Nova launcher, swiping in general, scrolling in chrome and settings menu
Thanks nJoy, I did a little more digging and I can confirm what you are saying.
Apparently what you can do is fastboot flash .img files e.g. system.img. These usually come from a backup or a google stock image, so flashing any other ROM needs a custom recovery i.e TWRP.