We have some new experience farming methods coming with the Sins of the Horadrim!
The XP Affix
Season 9 brings new Nightmare Dungeon sigil affixes, all of which you can find here. They have the usual negative affixes, but some also have positive ones. One of these is the Forgotten Wisdom affix. It makes mobs drop orbs, which grant experience based on your level.

Unfortunately, you can’t really make good use of these on your first character, as you’ll have to stockpile a few with that specific affix to get things going. They’ll still be a nice find if they drop, but one dungeon isn’t going to get you to 60. But 6.5 will!
A Solid Way to Level Alts
CulturalNinja6 took a poke at leveling up a second character using only these NMDs. After gathering up enough sigils with Forgotten Wisdom, the testing began. It took one and a half hours of not extremely optimized dungeon running to get the new character from level 1 to 60. Six and a half dungeons were run in succession, clearing pretty much everything in them, since it’s the monsters that drop the extra XP.
There’s even charts of the level progression, as well as how much XP the orbs give based on your current level! It seems the peak is around level 30, where each orb is around 7% of a level. As for endgame paragon farming, the orbs give 1.68% of a level at 60.
Now, the speediest way to level alts is still gathering Whisper Caches from Torment difficulties and saving them. This method usually gets a new character to 60 in under a minute, but you’ll need a solid chunk of the caches, as you can see here.
So if you want to save your caches for your main or are just looking for a different way to level an alt, keep an eye out for those Forgotten Wisdom dungeon affixes and save them up!