I've noticed that my iPhone 5 has lagged a few times when I would be in the text messaging app and I'd be receiving some text messages.
Earlier today I also clicked the sleep/wake button to wake up the phone, and the screen would stay black, and then I'd click it again and the home button as well, and it wouldn't respond for a few seconds.
I'm not sure if only i'm experiencing this problem, but when I first set up my iPhone, I set it up from the iCloud backup of my iPhone 4. So I'm restoring it right now to see if it helps.