I think my biggest complaint right now about performance is the speed of the onboard storage. The 5 is wicked fast at doing photo edits, but if I add a layer (a save operation) or export an edited photo (a save operation), it's just as slow as the iPhone 4S. My bet is that this is why they didn't go with a larger 10 or 12-megapixel camera - they'll save that for the 5S, when the onboard SSD can go faster.
I'm also kind of peeved about the USB 2.0 connector on the new Lightning connector. Calling it "Lightning" and leaving it as USB 2.0 is a bit misleading, sort of like racing stripes on a Geo Metro

I regularly dump 10 or 20 gigs of photos and videos off my iPhone and it sometimes takes over an hour to transfer all those files over, which is kind of annoying because then my phone is tethered to my computer during that time instead of being useful