Action Cooldown Equation FAQ
This isn't a complicated FAQ, but it's good to know if you're wondering about how positive cooldown rates actually works so that you don't overspend on skill points and overequip mods with cooldown rate bonuses.
I originally through that relic of a cooldown rate of +25% would cut off the cooldown rate of the Action Skill by one-fourth, but that's not how it works. So after a little experimentation, here's the actual formula:
Total Cooldown (seconds) = Base Cooldown / Cooldown Rate - Cooldown Bonus
The Base Cooldown is indicated within the Action Skill description of each character's skill tree. For example, the Commando's Base Cooldown is 42 seconds.
The Cooldown Rate starts at 1.00 and every percentage point of bonus cooldown rate increases this number for 0.01. So if you equip a mod with a cooldown rate of 28%, the Cooldown Rate is 1.28. All bonus cooldown rates are additive.
The Cooldown Bonus is exclusive to the Gunzerker, who has the Get Some skill in the Berserk branch of his skil tree. This lowers the Cooldown by 0.6 seconds per hit (once every three seconds). This bonus is of course subtracted from the total cooldown.
So, for example, if a Commando has a character mod with +30% Cooldown Rate, 4 points in Resourceful for a +20% Cooldown Rate, and a Proficiency Relic with +25% Cooldown Rate, then:
Total Cooldown = 42 seconds / (1.00 + .30 + .20 + .25) = 42 / 1.75 = 24 seconds.
This effectivelyreduces the Sabre Turret's cooldown by 18 seconds, which is a hefty bonus.
On further analysis, though, each percentage point of bonus cooldown rate has a smaller and smaller effect. In the above example, if the +25% Relic was unequipped, it would only increase the total cooldown to 28 seconds. This difference in 4 seconds might not be worth the mod slot at all.
However, I think it's worth gettting up to a +25% cooldown rate overall, particularly for the Commando and Mechromancer who have very long base cooldown rates. For both these characters, it would effectively reduce the cooldown by more than 10 seconds.
Of course, it's up to you whether you think Cooldown Rate is that necessary. It may be more important for you to increase the duration of the Action Skill instead of reducing the cooldown time. You might even consider doing both.