I know the mini is fine with one 4K monitor (at least with no scaling, or using the 1080p scaling mode), but wondered how it holds up with two?
I've got two 4K displays connected to i5/16GB mini right now.
After reading various posts about how the new Mini couldn't handle 4K, I gave it some testing, opening up lots of app windows, changing the scaled resolution to all the difference sizes, scrolling up and down, zooming apps in and out of the Dock, using the Mission Control key to have all the apps zoom in and out on the screen at once, etc.
Works just fine.
Everything is smooth, no screen stutter. RAM usage under 8GB, so while most users with two 4K displays are probably going to want more RAM anyway, it's not like you
need 16GB for two 4K displays.
Seems comparable to my 2013 Mac Pro (I believe the D300 is at least twice as powerful as the UHD 630 iGPU). I only have two 4K's handy for testing, but I did do a side by side comparison with just one 4K display on the mini and MP, and can't
definitively perceive any difference in performance, though if I examine
extremely closely, I think the Mac Pro might be fractionally smoother, but that could be my own perception-bias - I can't say for sure without doing a blind test.
I can't compare to let's say a top end 5K iMac, but my impression is that anyone "noticing" issues either has a faulty setup or is just the type of user who goes looking for issues... and there are a lot of those types on these forums.