The reason why Pro Tools is lagging on a 4-5k display is because the native resolution of Pro Tools currently supports 1080p. The 4-5k display feature is yet to be added by Avid. The current workaround for this would be switching to 1080 display so Pro Tools wouldn't lag.
I am using iMac 17.1 (5k Retina) and it is almost impossible to work with Pro Tools unless I switch to 1080 screen.
4-5k graphics support please!
Often (not always) when near the edge of a clip using the multitool instead of resizing the clip it will behave as if the grabber tool is active and move the clip, despite the pointer tool indicating the edge tool. This started happening since some time after PT10 I think and is still happening in 2018.1.