The iPhone 5 appears to be exhibiting odd touch behavior when it comes to consistent detection of multiple swipe events over time. On an iPhone 4S, you can begin rapidly swiping back and forth and iOS will easily keep pace, scrolling the view to match your mad swiping, even as you start angling towards the diagonal. On an iPhone 5, however, while vertical scrolling remains fine, rapid diagonal scrolling (approaching 45 degrees) will drop off, as though iOS simply tires of detecting the gestures. The issue was first reported by CMA Megacorp on Twitter, noting:
Slide finger back and forth diagonally on screen, input events drop out or stop altogether.
I was able to reproduce the behavior on an iPhone 5 with Mail.app, Contacts.app, and Tweetbot (though a couple of times in Tweetbot rapid diagonal swiping worked fine for without any drop off), Brushes (paint strokes stopped appearing), the iMore app, and more. I was also able to reproduce the same behavior on an iPod touch 5 using the same apps. Various devices were running iOS 6, iOS 6.0.1, and iOS 6.1 beta. That means it's not restricted to one control, like UITableView, or one version of iOS 6.
I was not able to reproduce the behavior at all on an iPhone 4S or iPad 4. That may indicate the issue is unique to Apple's new 4-inch, 16:9 in-cell display devices and/or the software driving them.
Due to the angle, rapidity, and consistency needed to trigger the drop, it's not a problem most apps or developers will ever face. Games and game developers on the other hand, could well encounter it.
Recombu produced the following video highlighting the behavior.