Will it support it without a discrete GPU, or should I get the 15 rMBP with the discrete GPU? Will there be any lag, like can be found on the 5k iMac with another screen? (I've even read that the 5k iMac GPU is so underpowered it lags while using Mission Control when no external screen is hooked up) I was looking at getting dual 24” Dell 4k IPS 60hz screens and either running them at native resolution or 1080p (HiDPi mode). Does it matter, performance-wise, if I run at HiDPI (1080p) or the full 4k resolution? If it will lag, what about just one 24” Dell 4k ISP monitor, would that lag? (Assume I am using the rMBP screen as well). What about one 32” 4k IPS 60hz screen (e.g., BenQ or Dell), and one 24” 4k IPS 60hz screen? Will my 15 rMBP lag while using the laptop screen, or with the laptop screen closed? If there is going to be lag, beach balls, etc, please just tell me and I’ll cave and buy a a Mac Pro. I can’t deal with lagging issues, despite much preferring the portability of a laptop… I REALLY, REALLY want the portability of a laptop, but the most important thing is no lag!