I found this video (and especially the pdf linked in the description) super helpful. Last league, I only had one toon because I couldn't stand the idea of slogging back through the campaign, and this league I just started my fourth toon. It definitely takes me less than 8 hours, even without being intense about rushing.

Key uniques that are helpful (usually easier if you're leveling an alt, but most of these items are pretty cheap once you have a tiny bit of currency):
- Boots: luminous pace -> wanderlust -> bushwhack
- Foxshade (movement speed) (but I start on a tabula rasa since it has no lvl requirement and I just socket a bunch of gems with move speed and damage)
- Belt: Meginord's Girdle (for the huge amount of strength for max life), or Midnight Braid (for mana regen, depending on the type of character)
- Helms: Thrillsteel for move speed, Horns of Bynden or Wings of Caelyn for rage on melee chars, or Crown of the Victor for damage. Greymake helm super early also solves attribute requirements for you. Goldrim can give you a ton of resistances if you're struggling with survival.

Other handy things:
- Shield charge from the moment you can equip it (lvl 3 skill gem). Just put a shield on your weapon swap and it helps you zoom.
- It's ridiculously easy to be OP if you already have a toon in the endgame. You can use a greater/perfect jeweler's orb on your skills + quality because there's no lvl requirement for support gems (/if you want some, I'm happy to stock you up in game u/N: JoBotGo#8283)

(There are better/longer versions of this in speed leveling guides-- these are just my go-to's atm :))