Classic WoW, Fastest Basic Method Of Leveling (Gamebreaking?), Self-Boosting Method, Requires High-Level Alt (Patreon)

There has been some controversy on the bug report forums about the rate of xp gained by essentially passive members of dungeon groups who are effectively being boosted by high-level players. 

Some consider the amount of xp game-breaking and unnatural: others state that this is how it was in Vanilla. For what it is worth I believe the mechanism was similar but subtly different in execution. 

That is as it may be: what really matters is what type of xp do you get from this strategy? I first tested this in a world with a low-level mage and a high-level rogue (I used two different accounts). I simply left my mage in relatively close proximity to my rogue and grinded mobs on my rogue. The xp gain was between one-fourth and one-fifth of simply killing an equivalent mob solo on my mage. 

Essentially the question now becomes: can you kill mobs 400% faster with a high-level character-inclusive of any drinking/eating time? Of course, you can, though I would strongly advise doing this with a class capable of aoe as your high-level character as my rogue ,which can only single-target in vanilla, was not at all efficient for doing this. 

There are two main types of location where this strategy can be practiced effectively:

1) Instant respawn locations. For example the dock to the south of Goldshire where a goblin dockmaster respawns along with three bodyguards every 15 seconds. 

2) Dungeons. 

It was clear from my limited experiment that boosting is going to be faster than any solo method, and that this is very likely to become one of, if not the best way, to make gold by charging for boosts. 

Now is this just a strategy for people with two accounts or those who can afford boosts? No. A player with a single account with one higher-level character on the same realm as a character they wish to boost can do the following:

Boosting yourself with a single account:

1) Open two wow clients from battle.net.

2) Get a random person to join a group with you. Sign up your high-level character then log-in on your level character, and sign up to the same group. The random person can now leave the group if they choose.

3) Take both characters to a dungeon or instant re-spawn farm, logging in on each as necessary.

4) Log-in on the low-level at the start of the dungeon or re-spawn farm and switch clients without logging out.

5) Log-in on the high level-if this has worked you should see your low-level character in-game. 

6). Kill everything in the area until the image of your low-level character fades-they will gain xp only up till this point. The low-level character fades after one minute.

7) Repeat 4 to 6. Reset dungeon if required.

(Veteran Patreons will recognize this strategy as being the same as my "method zero" technique which was very popular during the Legion expansion, before it was nerfed)

This is not quite as fast as being boosted by another player or multi-boxing but it is still faster than regular solo questing or grinding.

Update

So after reading Adam's comments below this article I confirmed his finding that level 1's are not getting any worthwhile xp when grouped with a high-level. Nonetheless I re-confirmed my own promising results: crucially this was based on a level 8 low-level character and a level 35 high-level character. What I think is happening: there is some kind of formula adjusting xp based on the ratio between a high and low-level character. This is further adjusted by normal factors such as npc level relative to the low-level character, whether the mob is elite, rested xp etc. 

I'm continuing my tests but at this time it seems that the above advice applies with the proviso that you level the character solo up to at least level 8.