Leveling is only fast when you get PLed. If you go by any other means it will take you time; spiritbond pace is also fine (at least with the current state of the game because there is no content for lv40-49 players (although endgame instances are 45+ no one will let you go in if not 50, and for good reasons))
But because you are complaining about the fast pace of leveling I'm going to assume that you despise these methods and are trying leves and other sort of options to level up and are still finding they are too fast. Would you mind telling us how much time it took you to get 1-50? Maybe try switching to just grinding solo and that might slow down the pace even more.