This is my personal feel on the matter. 1-39 speed is fine(this is with out pl cuz i wont do it) 40-50 that's stupidly fast.

The way the game works you can get skills from other classes so the devs have to consider a few things.

1.) They have to make the Leveling speed fast enough but not to fast so people are not discouraged from wanting cross class skills.

2.) They also have to consider being able to lvl more than one class to 50 on one char is very appealing so people will want more than one class to 50 unlike other mmos most people stick to one char.

With all that said i feel the speed of leveling is fine but they need to cap the exp earn in tiers for level ranges it pretty crazy you can get something like 2k+ exp for one kill.

1-10 max exp from a kill/chain bonus should be 200.

11-20 max exp from a kill/chain bonus should be 350.

21-30 max exp from a kill/chain bonus should be 475.

31-40 max exp bonus from kill/chain bonus should be 550.

41-50 max exp bonus from kill/chain bonus should be 750.

I feel if they did something like this they can control easily how fast people level.