Performance is very device dependent. I suspect it has to do with how the OS handles UIWebView. On IOS performance seems overall descent. Though I woulsn't suggest any intense physics until C2 binds to CocoonJS accelerated Box2D.
On Android however performance is a major hit or miss. My Acer A500 with a Tegra 2 is constantly a miss with performance for anything practically constantly between 5 to 30 with sometimes slower than the browser. However I've heard others with other devices get performance at 30-60 range.
Hopefully some adjustments will be coming in the next few weeks or month that overall improves performance :)