External monitor for Macbook Pro Retina 13" 2015 laggy (UHD like P2415Q)
I had a similar problem - it is due to your MBP not being able to "drive" the larger monitor.  It also has to do with how you are connecting to your monitor.  When I tried to go from mDP (mini DisplayPort) to DVI, there was lag.  But when I went to mDP to DP, the lag was gone.

The UltraSharp monitor that I am using is this model:  U2312HM UltraSharp IPS Monitor

I found this on Apple's Support site.  If you notice the resolutions and frequencies for HDMI go down as the resolution goes up.  This also applies for the other display formats.  You really can't drive anything more than 1080p at 60Hz  the more Hz, the smoother (less choppy) things look.



I will bet that if you tried this monitor on a PC with dedicated graphics, you would see no performance lag.