My guess is that the lag should be the result of too much heat being generated and causing a downgrading to the working squence of CPU. checking out from the ifixit, the tear down of S4 showed that the most of space of S4 has been occupied by battery and so the layout of components has to be squeezed into a very limit area on mainboard, in doing so, the only choice for this design is to use the panel as the sole thermal solution which is believed very unefffective and definitively will greatly harm the panel in a long rum. The HTC One, on the other hand, employs much smarter way to deal with the thermal issue, in which they used a copper foil to cover the heating components and thanks to the metal chassis, the heat can be evenly, and effectively transferred and evaporated. I hope my analysis is not true, or otherwise it will be a catastrophe for GS4, as I don't really think any firmware can resolve it unless the users agree to suffer a permanent downgrade for a much lower working power of CPU.
You'd notice it in the benchmarks, though, or during CPU-intensive tasks. This is a software issue, really, and a small one. The only thing that concerns me is that the sensors for the air gestures and eye tracking could be CPU hogs. Too early to definitively tell.
Sent from my Samsung Galaxy S4