Hi,
I've got a 2017 iMac i7, 512GB SSD with Radeon 580 8GB and 8GB RAM. A friend sent me some 4K video files and I can reproduce them perfectly in FCPX and Premiere, but for some reason Davinci Resolve Lite 14 gets choppy.
Funnily enough, my friend with a 2015 iMac i5, fusion drive, 1GB GPU...