Questing is the default and most common way of leveling in WoW Classic. You can play on your schedule, there is always content to do, and you are not reliant on anyone else if you are questing solo. If you do not have a set group for dungeon grinding, this can easily be more efficient and will minimize your downtime.
Your time spent while questing can be broken down into three basic parts: combat, recovery, and travel time. Combat is the time you take to kill enemies, recovery is the downtime while you recover Mana or health in between killing enemies, and travel is when you need to move between quests or zones. Lots of people focus solely on optimizing combat times, but the most efficient players will focus on optimizing their time as a whole. Often, this means focusing on increasing your recovery speed instead of increasing your damage, which is why Spirit is an incredibly good leveling stat for most classes. Some classes benefit more than others though, since classes like Hunters do not need the regeneration as much as a Warrior might while questing. Check out the class leveling guides to see what is best for your class.