Speed is mostly irrelevant to me compared to HOW you get to max level. Whether slow or fast, there just has to be enough stuff to get the job done without having to mindlessly grind.

And after your first class, seems most people group up and power level, which essentially means getting someone else to grind for you. I can tolerate that for a while but I know most people wont and the design of it is laughable. There should be some kind of mechanic or gameplay element to help make leveling a second class less boring once you've done all of the quests from leveling your first class. This could either be in the form of experience boosts that you can earn at max level and apply to your secondary classes, or just more content that can be done more than once, unlike quests which are a one time thing.