Using /effect <name> 3 10000 5, once the effect (Haste) is activated, the arm "hitting" animation fails to fire. Haven't tested this in MP, but I would assume the same there, as well.
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Torabi
Duplicate of MC-10755. Haste is normally only available up to level 2, and its effects above that are not guaranteed.
Once the effect wears off (I tried
/effect <player> 3 10 5) or the player dies, the animation fires as expected again.