Edit: Ability Haste not Spell Haste
CDR is a bit funky. It's the only* stat in the game that scales hyperbolicly. That's because unlike every other stat in the game, CDR does not add up but counts down. 10% CDR is actually 90% Cooldown. 20% is actually 80% and so on.
Like Riot said in their post, 90% Cooldown is not 10% more spell casts, it's +11%. (100/90)
Similarly 20% CDR is 100/80 or +25% or 1.25x as many casts.
| CDR | How many more spells you actually cast (Ability Haste) |
|---|---|
| 10% | 1.11x (11% Ability Haste) |
| 20% | 1.25x (25%) |
| 30% | 1.42x (42%) |
| 40% | 1.66x (66%) |
| 50% | 2x (100%) |
| 60% | 2.5x (150%) |
| 70% | 3.33x (233%) |
| URF | 5x (400%) |
| 90% | 10x (900%) |
| 95% | 20x (1900%) |
| 99% | 100x (9900%) |
| 99.9% | 1000x (99900%) |
| 100% | Infinity (Infinity) |
And if you notice, it goes up faster. This means your first 10 points of CDR barely does anything for you. But going just from 40% to 45% by taking the Cosmic Insight rune takes you from 1.66x to 1.81x or +15% ABILITY HASTE.
Now let's look at it the other way. How much Ability Haste do you need to buy to get an old amount of CDR
| Ability Haste | CDR Equivalent |
|---|---|
| 10% | 09.1% CDR |
| 15% | 13% |
| 20% | 16.7% |
| 25% | 20% |
| 30% | 23% |
| 35% | 26% |
| 40% | 28.5% |
| 45% | 31% |
| 50% | 33% |
| 55% | 35% |
| 60% | 37.5% |
| 65% | 39% |
| 70% | 41% |
| 75% | 43% |
| 80% | 44% |
| 85% | 46% |
| 90% | 47% |
| 95% | 49% |
| 100% | 50% |
Look how much Ability Haste it takes to get from 35% to 45% CDR. Ability Haste is objective power. 10% more Ability Haste makes you do 10% more damage if all you did was cast one spell over and over on cooldown. So why should the same amount of gold buy 11% power in the early game, but 25% power late game?
CDR left unchecked was a problem. And I'm glad Riot are changing it for Ability Haste, because given Riot's current trajectory, we were likely to see them unlock 50% or 60% cdr.
Oh wait they already did. Wind Soul. Cosmic Insight. Ultimate Hunter. Add/Multiply it all together with base CDR and you get 80.75 Ultimate CDR
Champions could already cast their ults over 5x more often if they built for it.
Yeah, CDR was busted.
