This strategy is about 8-9sph. (Seconds Per Horde)
The strategy is so overpowered for XP because most of the rounds are done with your Path of Sorrows specialist weapon, and this counts towards being a melee kill - more XP.
If you were to run the Tortoise Perk which causes you to not take damage when holding a shield, you can also just melee zombies with shield in the corner until your bored.

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