Most devices will charge at 3.7V @1.3 Amps (= 4.8W), and can pretty safely be stepped upto 3.8V @2.1 Amps (= 8W). However, some just suck, and the best you could hope to get out of them is 5W.

Newer smartphones are more capable and can use the full 5V @2.0 Amps (= 10W) but usually don't sustain it. They usually get a continuous reliable 8W charging though.

The top-end devices have "smart charging" so they can alternate the current to suit the need of the device (temperature, battery health, capacity remaining, etc etc). They use higher power at lower rates to manages this. They're limited to 15Ws, so they can start at 5V @3A... then shift to higher (thus more stable) voltages like 12V @1.25A (= 15W).

The fastest rate is on the Oppo Find 7a, which uses their VOOC technology.
In short, they keep the voltage and brute force more current. It can go 5V @5 Amps (= 25W) so its the fastest method yet. However, there are problems with this approach, like the charger can only be used for those smartphones, and rate itself is finicky. As the USB port gets worn out and the battery's health depletes, the system will cut back on that current rate more, and more often. So much so that it will seem like a fast charger but not really a "smart charger/VOOC". Some members reported this. The prevailing solution has been to put the smartphone into the freezer for 10mins before charging, into tricking it into hitting top gear.

My take?
I think 5.0Amps is outlandishly high. And that 12V is rather good approach, since its the universal voltage used internationally for cars, generators, caravans, boats etc etc. So perhaps we can see a 12V @2.0A smart charger, that would be a better balance. If charging gets unstable, it would be easier to drop the current and keep the voltage (eg 12V @1.0A) if need be.

Or you know, you could just have removable batterys...??