I have a 2015 MBP as well... it's HORRIBLE when I hook it up to a 4k monitor. Lag, freezing, etc. I can still get stuff done, but the experience is pretty bad.

I have a 2015 MBP as well... it's HORRIBLE when I hook it up to a 4k monitor. Lag, freezing, etc. I can still get stuff done, but the experience is pretty bad. Perfectly fine on its own screen though, the dual-core in mine just isn't up to the task of a 4k external monitor.