Leveling Speed?

[Niwashi]

After you hit 50 with one class you get a permanent bonus for any other jobs you want to level.

That doesn't come at level 50. It comes at level 10, as soon as you can unlock other classes. You'll only ever have full XP costs for one of your battle classes, whichever is currently your highest one. All others will get a 50% XP buff. (If you're leveling multiple classes in parallel, alternating back and forth between which is highest, that trades which one is getting the buff, but regardless, you'll still have it on all but one of them. Any time a new class takes over as your top one and loses that buff, your former top class gains it.)