The fastest way to level up fast in World of Warcraft is leveling gear.
Leveling gear is extremely important because the faster you kill mobs, the faster you do quests, the faster you do quests, the faster you level up.
Heirlooms don't give extra XP anymore, however, it still automatically scales to your level, so your gear will always be up to date.
The fastest expansion to level in is Warlords of Draenor (WoD).
The primary reason for this is that all the WoD zones have Bonus Objectives.
Essentially, they are one-time quest-type objectives that appear throughout the zones.
Completing these gives a huge amount of XP for the time spent completing them.
How Long Does It Take to Reach Level 70 in World of Warcraft: Dragonflight?
It will take you anywhere from two to 15 hours to reach Level 70 in World of Warcraft: Dragonflight.
The somewhat extreme time gap between those two estimates comes down to a few logistical factors.