I am not sure what it is, but the beta that Apple released today has helped my UI lag considerably. I am running a 13inch 2015 MBP. Both highest settings under Displays -> Resolution -> Scaled seem to run much better. I am not sure what has helped, I know they removed the discoveryd process and put back mDNSResponder but I figured that would only help with networking. Maybe the UI lag is due to lag in accessing network resources? Article: http://www.macrumors.com/2015/05/26/apple-discoveryd-replaced-with-mdnsresponder/ I am not sure but this latest beta seemed to help. Anyone else notice this?