Cooldown reduction is a special stat in League of Legends. Cdr has an out of ordinary pattern in nearly every aspect. I would like to point out some.

Gold efficiencyEdit

Most of stats have gold efficiency as flat gold value. Percentage stats (ex. % MS in Phantom Dancer item Phantom Dancer)  increase gold value per stat.

In most cases we can look at value of certain stat as relative increase too, ex. if champion has 2000HP and buys 500HP more, he has 25% increased value, if champion has 1000HP and buys 500HP it's 50%. From that point of view to maintain efficient development player should boost his already gained stats by others (health, armour, magic resist | attack damage, speed, critical, armor penetration | ability power, magic penetration, cooldown reduction).

From both points of view cooldown reduction is very special, because stacking cdr gives it more efficiency.

Example
  • You have skill with 10s cd. You can use it 12 times in 1200 seconds.
  • You bought Morellonomicon item Morellonomicon. (20% cdr)
  • Now your spell has 8s cd. (20% from 10s is 2s. 10s-2s=8s) You can use it 15 times in 1200 seconds (25% more compared to 12 times before).
  • You bought Frozen Heart item Frozen Heart. (20% cdr)
  • Now your spell has 6s cd. (20% from 10s is 2s. 8s-2s=6s)
  • You can use it 20 times in 1200 seconds (67% more compared to 12 times on basic cd, 33% more in comparison to 15 times with 20% cdr). Important to notice: percentage increase is higher then when one got first 20% cdr!
  • But! (8-6)/8*100%=25%! Wow, that Frozen Heart gave you not 20% cdr, but 25%!

What's important:

  • More cdr = higher gold efficiency
  • Higher cdr grows expontentially "in battle" strength

Hard cap and soft capsEdit

Cooldown reduction has hard cap in 40% cdr. It's only such a stat, Movement speed has soft caps, penetrations and resistances have unlinear effects. Champion cannot exceed that cap with purchases, if you alredy bought 40% you don't get more. It's important for supports, they have many valuable cdr items and they should avoid stacking them to higher then 40% cdr with items, runes and masteries.

  • Edit: Attack speed has hard cap too, thanks for pointing it out. In fact AS cap is much harder to achieve.

There is wide range of cdr soft caps affecting champions.

Some champions have long time effect, that becomes permanent with some cdr. Some are not affected by cdr at all. Examples:

As you noticed, 38% is not that far from 40%, so one can take 40% cdr in build and it's ok. But Mundo should avoid more than 30% cdr. Amumu and Warwick should get cdr despite they have toggle on/off abilities.

Cooldown is low already for some champions - probably the best example is EvelynnSquare Evelynn, she can spam her Hate Spike Hate Spike anyway and she should use some autos and move between. Player should avoid buying cdr for her. Edit: actually it is more important then many think. Animations, projectile speed and effects make cdr less rewaridng in many cases if cooldown is short enough already.

Resets: KatarinaSquare Katarina, Master YiSquare Master Yi. With reset mechanic it's totally wasteful to get cdr for those champions.

Resources are limiting many champions
  • Energy is limited resource. To commit full combo energy champions like KennenSquare Kennen or AkaliSquare Akali use up their whole energy. With cdr player in most cases must wait to replenish energy before throwing next combo even if cooldowns are up alredy.
  • Mana is not big limitation for many champions, but for example PoppySquare Poppy has big mana issues. Despite big utility from her spells it's much more effective to avoid cooldown in her kit.
  • Health can be problem for Dr. MundoSquare Dr. Mundo or AatroxSquare Aatrox.
  • Coolness. Joking, I am writing about RumbleSquare Rumble here. He can silence himself without cdr, with some cdr it is a disaster.
  • Lack of items. It's common issue among manaless (energy too) champions. Cdr is a great utility for them, but most of cdr items replenish mana too. It's waste. That's common reason why there is no manaless support too.

Spell effectsEdit

Math here, sorry.

Lets take for example Incinerate Incinerate. That spell has 250 basic damage on 5th level, 0.85 AP ration and 8s cooldown. With pure 40% cdr it would have 5.6s cd, that spell can be used 67% more frequently. 67% from 250 is 168. To have 168 more damage you need 198 AP.

40% cdr is equvalent to 198AP in this case.

Let's calculate Gold efficiency. 198 AP is worth around 4300g (198*21.75). 40% cdr is worth 1280 (40*32g). Seriosly, that 40% cdr you purchase with 1300 gold from items is worth like 4300 gold!

Not to mention you have better chance to use spell again if missed or provide effects like crowd-controls etc. That is seriously important. Obviously bronze player never miss Rocket Grab Rocket Grab (or he is called retard by teammates) but Korean players have tendencies to avoid skillshots on daily basis. That's why Koreans prioretize cdr much more.

With some AP, lets say 120 from Zhonya's Hourglass item Zhonya's Hourglass damage from Incinerate is already around 350. In that case to afford 67% increased damage one need to have roughly 280AP more. I won't calculate gold efficiency here.

Is that cdr much or no?

Actually, it depends on champion and his spells, ex. NautilusSquare Nautilus, SejuaniSquare Sejuani, or RammusSquare Rammus have spell effects like cc on nearly every spell, so cdr is very valuable for them. Casters (LissandraSquare Lissandra, RivenSquare Riven) tends to prefer raw damage for time being, but after some items and levels it's very good to get cdr. Marksmen rely on basic attacks, so cdr is rather waste.

SummaryEdit

Plan your itemization carefully if you want to get cdr!

  • Try to get 40% cdr if you already have 20%. Really.
  • Never buy more than 40% cdr with items, runes and masteries.
  • Try to get some cdr on every champion that can make use of it. In most cases it's better to have 600AP and 20% cdr than 900AP. (6 Rabadon's Deathcap item Rabadon's Deathcap TeemoSquare Teemo FTW!)
  • Avoid cdr on Energy champions, Health champions and reset champions.