Found this on another website...interesting!

July 10, 2005
Leveling Time By Class

Server Sample: RP (High), PvE (Medium), PvE (High)
Sampling Period: 6/13/2005 9:00 am - 6/20/2005 9:00 am
Sampling Resolution: ~14 minutes
Parsing Method: The sample unit is leveling event. We tabulate the time between a character's level and when we observe them at a new level. Only a player's online time is counted. We exclude the first leveling event from every character because it doesn't constitute the total amount of time to make that level.
Data Filter: None
Sample Size: 81,887 leveling events

We tracked the leveling time by class split for 3 level ranges: 1-20, 21-40, and 41-60. In all three groups, the starting level is controlled for. Controlling for group ratio produces similar plots. Druids are consistently the slowest levelers. Priests, mages, and shamans are consistently the fastest levelers. Between levels 1-20, mages level 10% faster than druids. Between levels 21-40, mages level 14% faster than druids. Between levels 41-60, mages level 20% faster than druids. The data shows that between level 41 and 60, mages make each level about 2-3 hours faster than druids. Even when controlled for group ratio, the trend is the same.

In fact, we know that this effect is not being driven by group ratio because Druids group almost just as much as Priests. At first we thought this may be a difference in DPS and downtime (which could make sense for shamans and mages compared with druids), but the priests throw us off. So it may be a combination of efficiency (DPS) and demand in groups that yield high XP.