Performance on an i5s and i7+ both run pretty well. But C2 did that pretty well too (talking 5ish games from C2 and 11ish C3 ready). Overall zero slowdowns from what I have personally experienced; but than again, I don't do over-the-top performance demanding games.
I think where C3 really has shined most is prep for the app itself. The XDK was literally a pain to work with, so many tiny details to get right or something will look off or just not function the way it did in test runs (html preview). C3 gives more hands on with xcode compared to importing to the Intel XDK and I like the logo/icon adding more now than adding every possible resolution known to mankind and getting errors from the Intel XDK.
Apples development app feels easier to work with and can upload directly to the store (which is super nice, archives are a godsend and doing all my work to a Mac, or remoting in and importing to xcode is easier than ever).