It doesn't happen on windows computers with Ultra HD resolutions. The problem is with Apple having not optimized OS X properly for their product before launching, and even still it's not fully optimized.
The rMBP lags on every resolution option. It is worse in some than others. If you were to grab the corner of a window, and resize it (make it smaller, or larger) and do that really quickly, you will see immediately the lag. If you compare it to a cMBP (old non-retina version) you will see that the cMBP does not lag whatsoever and it almost makes the rMBP look like it is the 3-4 year old machine. I was hoping Apple fixed the lag with Mavericks but they only made it slightly better
Which model are you using? Your experience is almost 180° different then mine (2012 2.7/16/768--Mavericks). I use 1920x1200 99% of the time and the 'lag', scrolling jumps and glitches on ARS, TechCrunch, Facebook, as well as a couple of other 'busy' sites is essentially all but gone. Lion wasn't great. ML made a big improvement....Mavericks, IMHO has definitely squashed ANY & ALL lag issues (which were never half as bad as many will lead you to believe) have been dealt with in the OS itself
I've noticed TechCrunch, ARS and others have 're'coded their sites for HiDPI displays. Again, IMO, that's where the 'issues' were. Heavy saturation and poor coding on the part of these developers of extremely busy websites. Tons of content. Lots of pics, video, Flash ads, etc. those are gone now and in a year....I'd say 90-95% of the apps I use from Adobe have ALSO been updated for high resolution support....as well as hundreds of others. It's been an amazing and FAST transition over the past year....as Windows is also stepping into the Hi.DPI arena. I'm hoping 2014 is that 'year'. Support for high resolution across the board (software) with their scaling code to match up with the working OS.
Apple can only do so much. For so many years....we've been using half these pixels. Apple's initial pixel doubling effort was a major achievement regardless of what, again, some will lead you to believe. ML and Mavericks has bettered their scaling engine even more....there's TWO sides to EVERY story. The other side, the coders for apps, sites and software HAVE to get on board too or there will be clashes
Mission control. Launch pad. Not sure where you're seeing frame drops or stutters but again, my year old rMBP is absolutely as fluent as I would like it to be. And usually I'm using the Intel 4000. A much lesser iGPU then the iris pro. I also use gfxCardStatus to force the 650 when I feel it'll help. In those cases, it handles troubled sites aplomb.
I can't speak for the 13"--- but as well as mine is running on the 4000 typical for most workflow the 5100 is a helluva lot faster. I'm sure you'll be good to go
With Photoshop though, I'd take a close look at the 15" and your finances though. The larger screen area is awesome when working through PS
Good luck
J