JavaScript execution speed is no longer a major limiting factor on mobile. In terms of UI JavaScript execution speed will help you if you're using jQuery (or any naive animator) to animate 2d CSS properties. The DOM, 3d transforms, etc are the real pain points.
As bad as UIWebView is, it somehow manages to be the best webview out of any mobile OS.[1]
I'm going to assume you're getting downvotes for saying that jquery animations are better performing than native CSS animations. From everything I've seen, that's simply not true, regardless of the platform.
I sure hope the downvotes aren't for saying that iOS's uiwebview is the best one we've got. Android web views mean using the android browser rendering engine, which is terrible. It might not be intentionally crippled like apple's is, but it is less capable to begin with.
I'm pretty sure I made no comment as to which is better, jQuery or CSS3 animations.
On mobile there is no question whatsoever that CSS3 animations are much better. The big issues tend to be regarding 3d stacking contexts. You can end up with strange artifacts, z-index breaking transforms, and sometimes downright wrong positioning. Also GPU accelerated elements can sometimes come out blurred. jQuery animations fix this but perform much worse.
On desktop it really depends. Browser compliance is an issue a lot of the time. You can actually slow down a 2d transition by using translate3d or any other GPU accelerated value. GPU acceleration hasn't had all its kinks worked out yet in browsers but its very likely that CSS3 transitions will be miles ahead for regular use cases in the future. I'm not saying that CSS3 transitions are worse, just that they are not better all of the time (only most of the time).
As bad as UIWebView is, it somehow manages to be the best webview out of any mobile OS.[1]
[1] as for BB10, I'll believe it when I see it.