Moratari's Infinite Wisdom
The bonus is given by the quest A Lesson in Alacrity, given by Moratari at the Infinite Bazaar. You can pick up the quest before you hit level 70, but it isn't necessary. Once you reach level 70, head back to Moratari and turn in this quest, giving you +250% XP to speed up the end of your leveling.
This bonus is additive to other Remix bonuses you've unlocked already from your Artifact Weapon.
The quest serves as a form of 'compensation' for the much higher XP requirements from 70-80. By obtaining this bonus, your leveling speed should be about the same at 70-80 as it was from levels 10-70.
