as you can see, you need more experience the more Perks you go for. also, since you go to level 1 after choosing a Perk, you have to level up to 99 one last time after choosing Perk 6.
this is why Ascending Builds are important, as you want Quest Stones on all Items for +200% experience. 3 Ascending Sets of level 1, 40, and 100. also, experience from monsters stops going up at floor 100, so you only need to kill monsters around floor 100-120. you only need higher floors if you are looking for certain items to improve your Ascending Build. there is an experience bonus for all difficulty levels, except for Very Easy.