I have a Samsung 4K Monitor (U28D590D) connected to my MacBook Pro (13" Retina Early 2015) through HDMI.
Without the Scaling option in the Display Settings menu, it works great, but to take advantage if it you need to scale. So, I enable scaling and in any scale the performance of the Mac degrades terribly. It becomes really slow and it is very difficult to use my Mac that way. The connection is direct (not via any peripherals), and there seems no way to improve this. Has anyone experienced the same issue and solved it?
MacBook Pro with Retina display, OS X Yosemite (10.10.5), 13"Retina Display