I'm not sure what it is, but I find Jelly Bean laggier than ICS on my GSM Galaxy Nexus. Sure, certain things are "butterier" and feel amazingly fast like when opening or closing apps or opening/closing the app drawer, but other things are strangely slower now. For example, when exiting an app to the home screen, if you immediately swipe to the next screen (left or right) there's this slight stutter. Never ever noticed this in ICS. I keep reading about JB still having micro-lag, and I'm not sure what that means, but I'm guessing that's what I'm seeing. And again, I feel ICS was smoother (not faster, but smoother; never saw ICS stutter). Really perplexing.
The keyboard has also seen a serious downgrade. It behaves very much like the iOS keyboard now. It misses keys, misses auto corrections (e.g. I should never ever see "ypu" when typing "you". "Ypu" should always auto correct to "you" but sometimes it appears in my emails/messages; it's like the keyboard forgets to or misses it and doesn't correct it. It happens frequently), and I really feel Google completely missed the point of shortcuts. It should not be offered as an option in the suggestion bar; it should automatically replace it with what you want. Having to pick the phrase you want of your shortcut from the suggestion bar just adds an extra step that is wholly unnecessary. I think the iOS keyboard is terrible, but if there's one thing they (and Blackberry) got right, it's shortcuts. I swear, typing on ICS was way smoother, more reliable, and less prone (almost never) to missed keys or missed autocorrections. There's also an incredibly odd glitch with backspacing after a period. There are members at the XDA forum who also agree the keyboard has slowed and the response time downgraded. Real shame, Google.
I'm really not sure what happened, but that boat that I said that was sailing for iOS suddenly made a u-turn. I'll be keeping a hopeful eye for surprises on the next gen iPhone, even though I'm not thrilled by iOS 6 at all, nor the rumored mere .5+ inch screen increase.
There are so many tradeoffs now between the two platforms. I foresee a pretty difficult decision to make depending on what happens this year.