They will always say faster javascript, it's probably definitely true but I wouldn't expect night and day difference.
One other important thing about iOS 6 (and desktop Safari 6) is remote debugging. This will be huge! You can do it with weinre but now it's built in. Not sure if you have played with the remote debugging in Chrome for Android where the debugging happens within chrome on your computer, it works pretty much the same for what I have seen.
For me the biggest improvement is support for the file upload field, even if it is limited to photo's and videos.
I've already modified the code of my app that if it detects iOS 6 it enables the file upload functionality (was only enabled on android and blackberry before).
I couldn't test it because the iOS6 beta SDK simulator is reporting the wrong user agent. I hope that ST will detect iOS6 correctly when the final release rolls out.