According to iLounge, the battery in iPad Air is 32.4-Watt-hours compared to the 4th gen's 42.5-Watt-hours, which will allow the device to charge faster and reduce the charging time to closer to 4 hours. If this turns out to be accurate I'll be very happy.
My iPad 3 is a dog when it comes to charging.
http://www.ilounge.com/index.php/news/comments/apple-quietly-changes-ipad-air-retina-mini-batteries/
http://www.ilounge.com/index.php/news/comments/apple-quietly-changes-ipad-air-retina-mini-batteries/