How to slow down leveling

My wife is a casual World of Warcraft player. She doesn't participate in any form of group content, neither dungeons, nor raids, nor PvP. In consequence there isn't much to do for her in the "endgame" besides daily quests, and thus she has tons of alts and spends most of her time leveling. And since Cataclysm, she isn't all that happy about her leveling speed: It goes too fast.The problem is a design philosophy of Blizzard, who apparently decided to keep the overall leveling time up to the level

World of Warkraft...