Correct...Default (non scaled mode). This is the only mode that works. I was hoping people have seen similar on Iris Pro as it appears via a search on internet that this is a common problem with 4k monitors as people moved to HSierra. Apple forums have tons of complaints, albeit no solution to problem yet
Erm, that's not what I asked. When you're running the other resolutions that are lagging, they're scaled, yes? In High-DPI or using the selector method in my screenshot? Because, as I stated before, that's High-DPI mode. If that's the mode you're using, that's normal behavior. You're pushing an image that's higher than what your Mac supports rendering @ 60Hz. To clear up any confusion, do me a favor; screenshot your desktop in the resolution you're having trouble with, right click that screenshot, and select "Get Info". What is the dimension of the screenshot?