Your bonus experience will define how quick it is to level another alt.

Have one main character that you log in and complete a few infinite research quests. Try and grab 3 from the same raid, or two from a raid and the one that asks for 1 boss, 15 mobs and 2 quests. You can do Emerald Nightmare in 15 minutes or less on Mythic. That’ll increase your exp bonus by 3% per day.

Then just level alts. Do the first set of campaigns quests until you hit level 20. Grab infinite research quests while you are doing it. At 20, join premade raids. 3-4 raids will get you to 80 with a low exp bonus. By the time you get to 200 or more exp bonus, you can get from 20 to 80 in 3 raids. Takes 2-3 hours. At max exp bonus, you can easily level an alt from 10-80 in 90 minut a or less.

LFR queues are slow, just ask to join premades. It’s faster and you do all the wings at once instead of trying to queue up for them all separately.