Depends a lot on the expansion - most recent ones are better, but in BfA you can get to 50 by the time you reach the third zone, doing only the main quest and not setting foot in the horde ones.
If you want to be fast (and WoD is king) just download Azeroth AutoPilot.
Edit: obviously this doesn't really apply to Vanilla, but the zones are built differently and are meant to be smaller/faster. There's room to level to 50 in multiple different ways.