That's strange. I use an early 2015 13" model which happily drives two external 4k displays simultaneously. I'm not running them high-DPI though – the performance there is noticeably worse.

I have a 2017 13" and while it was working with a single 4k screen at 60 Hz, it simply isn't fun working on it since everything is laggy and stuttering.

I guess that high dpi then as well?

I found that wasn’t really nice to use because of the stuttering, but native 4K was much better. You need good eyesight though :)

Scaled down to 1440p yep. Native 4K is no option :D

On scaling other than 2x, it's unusable, while the MBP mobile Radeon Pro is good enough. They should have included that at least.

Yeah, I was doing 2x4k plus the internal screen off of the MacBook with a 555, and it was totally fluid. I would have happily forked out for the Mini if it had that GPU; it seems like a bit of a strange decision for such an otherwise powerful machine!