It's all about energy
The most significant performance limitation of smartphones are their form factor. Passively cooling a chip inside a sealed case is a challenge, and the heat dissipation rate determines how much power can be sustainably drawn during gameplay.
For most devices, the System on Chip (SoC) power budget is between 2.5 and 3.5 Watts. This only leaves 1 to 1.5 Watts for the GPU. The challenge for you, when trying to get the best performance and rendering quality out of these platforms, is how to be as efficient as possible with that power budget.