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!