Buffs. Food/Flasks/Scrolls/Potions/Elixirs are a small, but noticeable, buff to your killing speed and survivability. If you have a max level character that can already supply you with cooking or alchemy, you'll be very well off, because the AH is a cesspool for this. I can't really give you specific foods or potions to look for, since each AH is going to have different supplies. Generally speaking, lower level foods are going to only last 15-20 minutes until you can buy the more powerful versions at 60+. Look for anything that adds versatility or your primary stat, and buy stacks for each level range(1-60/70/80/90/100).
As always, I will monitor and update this guide as new information comes in. If I've gotten anything glaringly and completely wrong, just tell me with as much data as possible. The idea is to find the fastest possible leveling method, NOT to argue about how good or bad individuals are. If you have hard data, post it! If you have an idea, let the thread know so we can test it!
The Heart of Azeroth is an artifact given to the player character from the world soul via Magni Bronzebeard. It is a necklace that can use the power of Azerite, the lifeblood of the embryonic titan Azeroth (See Plot, below). Azerite is similar in function to Legion's artifact power, in that it is gathered normally from playing the game and is used to upgrade an artifact; likewise, the Heart is similar in function to Legion's artifact weapons by having the capacity to be constantly upgraded. Unlike artifact weapons, however, the Heart is shared with all of the player's specializations and can empower all eligible pieces worn simultaneously.
The expansion brings a major change to the PvP ruleset on each realm. Every realm by default only allows players to attack NPCs in the open world; players who wish to engage in world PvP now have a setting called "War Mode" that can only be toggled on or off in their faction's capital city (i.e. Stormwind for the Alliance and Orgrimmar for the Horde). While in War Mode, players have access to new talents and abilities, as well as a slightly accelerated rate of XP gain. Characters with War Mode activated are only able to see other players with War Mode, unless in their own capital cities.
If you've been away from WoW for some time, you may not know that the game's Legion expansion focused on powering up an impressive "Artifact Weapon" throughout the duration of the expansion. In short, players have been using the same weapon for about two years now. Artifact weapons were pretty cool and bosses did drop Relics which were used to upgrade them. But after two years of using the same weapon, many players are looking forward to the thrill of having an actual weapon drop off of a raid boss again.
For reference, my previous best time abusing Method-0 in 7.3.5 was 32.4 hours(as show in the old thread). A difference of just over 8 hours; which isn't all that bad, all things considered. And I believe I made several errors along the way, which could be cleaned up in future runs. I believe that that gap could be significantly closed by using a monk's bonus, but I hate playing monk...so...yeah. YMMV