External monitor for Macbook Pro Retina 13" 2015 laggy (UHD like P2415Q) I have a Macbook Pro Retina 13" 2015 and I'm searching for an external monitor to use with it.

Until now I tried two external monitors: DELL UltraSharp U2515H (25") and Dell P2415Q-C (24").

The first one (U2515H) is 4K monitor and is amazing. A lot. But, there is a but.

The monitor is not sharp (retina, splendid, beautiful) like the second one, the P2415Q that I tried.

The second one has 185 PPI (pixel density: 3840x2160 on 23.8") that make this monitor amazing, beautiful, excellent, LIKE A RETINA DISPLAY!

But, another but.
My Apple Macbook Pro Retina 13" 2015 (yes, the last one, the one with 8Gb of RAM) is crappy and lag a lot using Chrome, Safari and other software in any resolution, INCREDIBLY!!! except the last one (3840x2160). I think 

because my Mac at the 3840 (4k) resolution doesn't have to manipulate and recalculate any pixel and so it is fast and usable. In other resolutions, also in the Full HD (1920x1080), it is crappy and unusable like this:

https://www.youtube.com/watch?v=EBgLMls-f6I

I hope this is not a fail of my own Mac, of my own graphic card. Because if just a my problem I have to fix this. Can you confirm this is not just a my problem? Just my pc makes like this? Can you post a scroll on a website with your 13 rMBP?

So, for this reasons, now I'm looking for an external monitor to use with my Macbook Pro Retina 13" 2015 which is at least 21" (max 22!!!) and Full HD (so my mac have just to be a division by two: from 3840/2=1920 and 2160/2=1080, and it will suffer less that a mathematical operation with height like 1440p or 1600p) but the monitor have to be an UHD one, at least high resolution like DELL P2415Q which have 185 PPI.

There is something like this? Thanks in advance.
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.