I bought this product 5 months back, due to the new touchwiz of samsung there is no lag till date I have notice I am hardcore gamer I play game for about 4 hrs continuosly and on my continuos usage.
I didnt find any lag or shuttering of games there would be no problem in playing games like MC5, Nova 3, asphalt 8, Clash of Clans.
In terms of battery backup it is the best playing games on full charge can give you screen on time of 6hrs and on moderate usage 9-10hrs standby really great and now days samsung is providing every month updates for bug fixes and this device is getting marshmallow update if need a beast for gaming with good backup and great camera go for it, it is worth the money you spend