I'll bet the entire os has debug symbols and other things turned on that never reach production.
That said, the jerky animations are annoying in use. But its mostly spit and polish things nothing overly broken. I did set this up on a 4s with a new install and not restore (i'm not putting a beta on my main phone).
Having said that I do use a Blackberry (but not really out of choice,) and it also drives me nuts!
I'd have thought that the number one priority for most of these UI developers would be to make something that was intuitive and very usable. Getting the phone part of the phone nice would be a good start.
Adding contacts, viewing call logs, checking balance, turning on and off answerphones etc.
Being able to read the display in sunlight. Being able to use it as a clock, and it be readable. Not having the thing beep at 4am to tell you that it's running out of battery etc.
There's loads of directions these UIs could go in. And they could be completely different to one another, but share some underlying principles. Like intuitive computer game interfaces, that you pretty much pick up immediately.
What I have been most impressed with is watching babies/toddlers play with iPhones. Which suggests the interface is pretty good. Though there have been cases of kids racking up huge account costs. But how about an easy way to child lock the damn thing, or make this harder.
And how about a phone that doesn't crack when you drop it. I watched someone destroy his Galaxy S the other morning, and he wasn't that pleased about it.
Hope I've suitably ranted for you.
This sort of lag has been a common artifact throughout the iPhone's history. Crashing apps, lag, and outright unresponsiveness are daily occurrences on my iPhone 5. It's honestly gotten to the point where, if I didn't have an astronomical amount of money tied up in their ecosystem, I'd switch to a Nexus 4 tonight.