If you combine Zerk's whirlwind, the yellow potion and the running ability, the hero gets insanely fast.
I suggest lowering the amount of speed obtained by reducing 2nd and 3rd speed buffs. Something like regularSpeedBonus*0.75^(n-1), where n is the nth bonus
The first bonus isn't changed (the formula outputs 1), 2nd bonus is reduced by 25%. 3rd bonus is reduced by 43.75% etc.
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