Level with a pet if you can because it will help keep you safe. If your pet is tanking all of the damage from the enemies you’re fighting, you won’t have to. The other reason is that pets are easier and cheaper to heal than you are. If you don’t have a pet, make sure you know how to use your defensive abilities. You can’t level quickly if you are dying all the time.
Last I wanna share a curious bug that almost freezes my game and force me to disable the addon. Anytime I add a (Citadel Crossbow [of the Fireflash]) my fps go from 75 to 22, and It's on a paladin, paladins dont even use crossbows. I noticed everytime I put something in my bags the addon takes the math and my fps goes from 75 to ~22 for a while (3 to 7 seconds) and is think it's internal and also think it's fine.  

Determine the gear stats and armor type you need for your new character. For example, a paladin requires Strength as their main stat, and heavy armor (the armor type) will scale as the character scales if they are initially unable to wear heavy armor due to being a lower level. Your optimal heirloom gear set will feature the same main stat as your starting gear.
I just leveled this char in about 2 days played time with a few hours of it being afk in cities.What I did was run every single dungeon one time and do quests while waiting in queue.That way you don't get bored and the experience gain is great.Works particularly good between 60 and 90. WoD is a piece of cake if you just have flying and Legion is questing + doing the per zone dungeon for the 100k xp quest.
Vol’Dun is the hardest and most tedious of the Horde Zones in Battle for Azeroth. It’s also probably the one that makes the most sense. Each sections of quests leads directly to the one you should do next. For all of these reasons it’s going to be best to get this one done early. Thanks to any legendary items you might have collected in Legion, you’ll be strongest in the leveling process before 115 and weaker after — because that’s when the legendary effects stop working.