If you want to spend as little time on your alt as possible - just do Legion Invasions. You can use sites like (https://wow.gameinfo.io/invasions) to track when the next invasion will be. Log in, fly to the closest world quest, and once there - right-click on the quest's name in your quest log and "Find Group". Sign up for some group, or if there's none - just start your own immediately, and people will come to you. It is 10 times faster to do invasion quests in a group. Once that particular quest is complete - leave the group, and fly to the next one, where you should find another group. Don't expect the group from one world quest to follow you to the next. Then you simply log out until next invasion. You can do 1-2 invasions a day, and each invasion will give you about a level or 1.5 of experience, if you have full heirloom gear. Each invasion will take about 20 minutes.
In my opinion, Zygor makes the best WoW leveling guides. They have been around for more than 10 years, which shows their commitment to maintaining and updating their guides to provide you with an optimized leveling path for any class and race, whether it’s Horde or Alliance. Zygor is an addon that includes a waypoint arrow and a separate window with instructions that, taken together, show you step-by-step how to level your character to 120, FAST. You’ll know exactly what to do at all times. It’s also very easy to jump in at any level; you don’t have to start exactly at level 1 or 110 or anything. It’s a bit more expensive than the other options, but I’d say it’s definitely worth it.
To give you some perspective I leveled both a LF Ret Paladin and BE Rogue (so one started from level 20 and the other from level 1). I focused on getting the achievements I was missing for the Loremaster, so not the most efficient route by any means. I was using full enchanted heirlooms (minus the ring) and the WoW-Pro addon though, as well as having War Mode turned on all the way.
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