old thread I know, but I'd like to throw my 2 cents in here as well
MB Pro 13 inch 2017 fully spec'ed (3,5 GHz Dual-Core Intel Core i7, 116 GB 2133 MHz LPDDR3, 1TB SSD, Iris Plus 650 1.5GB Graphics) attached to a 5k2k LG Monitor (34WK95U)
works as expected and no problems whatsoever. few comments though:
- native resolution not doable for me (5120x2160@1x)
- default for the screen 2560x1080@2x looks awsome but the interface looks a bit to big
- scaled "more space" 3360x1417@2x looks perfect size wise but although sharp I still feel is not as sharp as the default one.
i keep going back and forth between scaled and default and always I'm amased by the sharpness of the default. but i have to admit i'm still not sure if it's also 'cause being bigger I see better
anyway getting back to performance i looks like scaled really eats in performance. 4k youtube in scaled is choppy while in default plays butter smooth so i can only assume that this is the impact in performance highlighted in the settings (basically it drives a 6720x2834@1x resolution scaled back to half of it)
i thought egpus and stuff but don't really need them. i'd like though to run 4k youtube w/o the fans maxed out or to watch 4k60p youtube but there an egpu would not help due to VP9 which is CPU decoded on mac (AFAIK)