It doesnt happen much for my iPhone 4S, most current iOS (i dont remember what current version is, 5? 6? 8?)
It does happen on my iPod 4, previous iOS
I think it has to do with loading other players? The freezing happens most frequently when a) I first log on, b) enter a tower or other crowded level that I havent been in since logging on (after the first freeze up, it usually doesnt happen again in same level), c) when a new player you havent seen since logging on comes on your screen (i noticed the freeze ups start again mostly when a guild that wasnt there before attacks a tower, or when new pkers enter the tower).
The only thing that is against this theory is that I dont experience a crap ton of freezes in Graal City. Yes, I freeze a lot, and players take a second to load in graal city, but its not like absolute hell or anything that I would expect.