I tried the rMB again -yes again- in the Covent Garden store. This time I was pleasantly surprised and the evidence suggested where the main problem is Yosemite's UI. * I ran some benchmarks using IPython on a rMB and rMBP13. Array multiplication thousands of times. The times were similar at 500-600ms per loop. * I installed the GIMP photo editor and played with a few operations like curves and image warping and applying textures. Fluid and fast enough not to notice. So sounds really good!? Except that damn Yosemite is spoiling the experience with a laggy in interface. Damn shame. Because it would be really good otherwise (except the keyboard). Why does my nexus 5 have a more fluid interface pushing 1280x1024 pixels? Why does my netbook from 4 yaers ago with windows have a snappier desktop? Debloat that OSX and you'll have a fantastic creation. Hardware is getting better. Software isn't.