I have moved to xperia z2 (from lumia 920) and I strongly disagree with you. Over last weeks my phone has lagged zero times, no waiting, no stuttering, no '...loading..', no nothing that could be at least a bit similar to performance issue, and thats on a 2 year old phone... My mum's 640xl while being relatively 'new' has already been hard reseted once, because of performance issues, not event talking about 920... So my offer to you is: stop using cheap samsung (not The galaxy s line) or other cheap android phones and invest in better phone... Also if the lagging becomes unbearable try thinking about how bad wm10 would if at all work in your phone.