This is more or less an extension of this thread. I can verify that at least in my case, this is specific to OS X.
Under OS X, it seems that performance can slow to a crawl. My configuration in particular is a late 2012 27" iMac with a GeForce GTX 680MX on OS X 10.9.2. I’m aware that the OS X and OpenGL side of things hasn’t quite received the same amount of attention as Windows and Direct3D, but I thought I’d just post this up here in the feedback forum to make sure that the guys at Epic have a central thread to reference for OS X related perf. issues.
I’ve been doing a bit of profiling with Instruments and Xcode. I’ll post what data I’ve managed to collect at some point.