The iPad 3 eats power 70% faster than the iPad 2. iPad 3 with A5X processor (GPU power consumption doubled) + retina display (pixel density quadrupled) + max brightness + power-hungry app + Bluetooth + WiFi = power consumption > charge rate.
To ensure a similar battery life of 10 hours, the iPad 3 was given a battery with 70% higher capacity than the iPad 2 (42.5 Wh vs. 25 Wh). The same 10W charger therefore takes proportionally longer to charge it (~5 hours vs. ~3 hours).
So: why wasn't the iPad 3 given a 17W charger, to charge it as quickly as the iPad 2? Answer: heat issues. Cranking up the charging rate would lead to overheating.
