I have the previous gen late 2013 13” rMBP with 8GB of ram and Intel Iris 1536 MB graphics. The graphics performance is terrible…I regularly keep 15-25 Chrome, Preview PDFs, Textual and Sublime Text code windows open at once and I am regularly experiencing lag. I never game and rarely edit photos in PS. When I use my Apple Thunderbolt Display, when using spaces or mission control, the windows can lag. I switched from a late 2013 15 rMBP with discrete GPU because the new 13 rMBP was supposed to have better battery life. Well, I am lucky to ever get more than 4.5 hours of battery. My question is this: 1) How does the new 15 or 13 rMBP compare to the late 2013 model? 2) Is El Capitan supposed to help the software glitches with GPU performance? I am running a DP and have just noticed bugs but no real improvements vis-a-vis graphics. 3) I would like to get a Dell P2715Q 4k monitor and run it with my 15 or 13 rMBP. Is either machine/spec better for this than another? Can it run without lagging? Can I run two, plus have the rMBP screen open or does it need to be in clamshell mode? 4) Specifically, if I go with the 15 rMBP, is the discrete GPU actually needed for my needs since I don't game? It's a $500 add on which is significant. Similarly, is the 13 rMBP with Iris 6100 graphics better than the graphics card in my 13 rMBP? 5) Is the battery life in the new rMBPs actually better? 6) I would prefer to keep the 4k monitor and rMBP monitor in retina mode, will that decrease the graphics performance? If I make the resolution slightly larger for my poor eyes, will that help or hinder the GPU? NB: The 4k display(s) MUST run at least at 52hz, but preferably 60hz, otherwise, I do not want it. I REALLY do not want to buy a Mac Pro to be able to use 4k monitors without it lagging for basic tasks as I don’t want a laptop plus a desktop as primary machines, but it seems that is what I need, if I want to run 4k monitors. Hopefully the new rMBPs are better? Please advise.