Action Skills cooldown at a rate of 1 seconds per 1 second (wierd sounding, I know). Cooldown rates increase the amount of seconds per second. So, in other words, at 100% cooldown rate bonus, your Action Skill has a cooldown rate of 2 seconds per 1 second and, therefore, a cooldown of 21 seconds. 100% cooldown rate does NOT give your Action Skill a 0 second cooldown.
So, in other words, your cooldown rate is 1.15 seconds per 1 second with it equipped, which is why it's hardly noticeable.
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