The 2.5GHz Core i5 in the 13-inch Pro offers terrific raw CPU performance in benchmarks, running Geekbench at a solid 6700-6800 range, but the integrated Intel HD 4000 graphics chip can struggle driving such a high-resolution display. Oddly, it showed up for me more during day-to-day usage than under any crazy test situation I came up with: RAW files in Aperture scroll around just fine while QuickTime is playing back 1080p movie trailers, but Safari and Chrome both stutter a little while scrolling simple web pages. And they stutter a lot with image-heavy sites like The Verge and Polygon.
Youll also notice some general lag when you start multitasking heavily. Open more than a few tabs and apps and youll start to notice things slowing down around the system as a whole. My standard workday set of 15-20 tabs open in Chrome, music playing in Spotify, email and IRC open, and Skype, iPhoto, and Messages running in the background never pegged the CPU meter, but I could clearly feel the system running just a hair behind me. You might not notice it if youre coming from an Air or a much older MacBook, but Im used to my 15-inch Pro with 2.3GHz Core i7 and Radeon HD 6750M graphics, and and the 13-inch Retina is definitely a little slower. If youre a pro looking to step down to a smaller machine, youll almost certainly notice the performance dropoff as well.
The simple fact is that Retina display is a lot of pixels to push around for that graphics chip just consider that Apple uses the same GPU in the MacBook Air, which has a third of the pixels and can still be made to drop frames under heavy multitasking loads. Plug in an external monitor and youll feel the hit come even quicker in day-to-day use I tried both my 1080p TV over HDMI and my 24-inch 1920 x 1080 display over mini DisplayPort, and I could make YouTube videos stutter on one display simply by loading The Verge in the other.