I've searched around and couldn't find any good threads about this:

My note 4 (N910U) gets pretty hot when I'm playing regular games (Tsum Tsum, Wind Runner) and I'm thinking that this is what is causing it to lag.
I.e. When I start playing everything is super smooth, and then about 2-3 minutes in when the phone is getting hot, the frames start dropping and it lags pretty bad.

This happend with my note 2 as well. Is it normal or should I try to send it in to samsung somehow? (in which case does anyone have any experience with samsung customer service? I bought the note 4 from HK but I live in Japan :/)