Grinding mobs is tedious and slow. Questing is much faster since you get the massive quest experience in addition to the monster's experience. There is no grinding in this guide - you can go through every single level by simply questing! There is minimal traveling time as the guide leads you visually through maps so you can quickly rack up quest experience.
Take breaks. Unless you're straight up no-lifing it over a weekend or something, this leveling process is going to take ~40 hours. But any time you do need to step away, log out. That tiny little bit of rested exp is insignificant...but you never know when something might come up and you'll have to leave for a longer period of time than you expected. Better to be logged out and safe(especially with War Mode on) than AFK waiting for a d/c. Plus, you'll need to stretch, eat, shower, and maybe sleep. XD
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Special note: At 110 it's almost certainly worthwhile to go and immediately get your Heart of Azeroth and first piece of Azerite gear from the BfA opening quest. It doesn't take long and provides pretty high power levels. I'd then return to Legion quests until 111 or even 112, since the power of these items will increase your kill speed significantly over the scaled enemies of BfA content.
In recent weeks, players have been giving Blizzard an earful concerning experience gains and leveling in World of Warcraft. It looks like a couple of issues are at the heart of these problems, including time to kill certain baddies and the time it takes to level up between levels 60 and 80. While Blizzard is still trying to figure out why some baddies are taking too long to kill, the hope is that a hotfix launched this week will address the problem of leveling.
You should equip new quest rewards when their item level starts beating out your previous set of gear. If you are currently wearing Tier 21 gear, it is best to let the quest rewards significantly beat your existing item level before using them. With that said, you will unlock Azerite pieces early on, which are worth using over Tier gear in most circumstances.