Nice. Might have to upgrade from my 4S...
1. I don't think any of the episodes you tested have particularly heavy subtitles. 720p 10bit works on just about any modern hardware nowadays, even netbook class stuff, but subtitles can bog any system down if heavy enough. The only heavy subtitle in recent memory would be UTW Shinsekai Yori 3 prologue -> part a transition. Can you try that?
2. All of the episodes you tested are 720p I think (airing stuff usually isn't 1080p). Try some 1080p?
Anyways, in Fluff's defense, it would be a good idea to note that the iPhone 5 is about twice as fast as any iOS device preceding it. Things change quickly in tech, saying that TheFluff was spreading a rumor would be like saying all the people saying that the iPhone can't copy paste when it still couldn't copy paste were spreading rumors. It's not a rumor, it was something that was true then, but isn't now.
Can you back this up? The iPhone 5 is twice as fast as the iPad 3, AND has less pixels to push.