Mobs are not affected by debuffs on attack speed
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Tried with Slow and cobweb. All those effects reduce the attack speed for characters, but seem to have no effect on mob attack speed.
This is probably due to the fact that the slow effects slow down the animation of the attack, and characters are in a constant attack animation when at full attack speed. Mobs instead seem to do attack - wait - attack - wait. Slowing their attack animation wouldn't impair their attack speed if it doesn't affect the "wait" part.