4K Monitor and performance issues with MBP Late 2015
Scaling is your culprit. Anything different than 1080p or 4K is going to tax the GPU and cause lag. It's unfortunate. I think a solution which costs a lot more money would be to purchase a 5K monitor then run it at 1440p (the only acceptable resolution imho) This would be the default resolution since it's exactly half of 5K. 1080p is exactly half of 4K which is why the GPU doesn't struggle on a 4k monitor when run at that resolution. Of course 4k at it's natural resolution is far too tiny to be useable which I'm sure you found out. So 1080p and 4k are just not useable for a 27" monitor. Purchasing a blurry 1440p monitor or a 5K monitor run at retina 1440p are the only options to get a lag free experience imho on a 27" external display paired with a MacBook Pro.