Do not use Celerity rune after next patch - It is dramatically worse than it looks (maths inside)

THIS IS NOT MY WORK, IT IS FROM A REDDIT POST HERE: Thanks to Reddit user bazopboomgumbochops

For those who don't know:

Celerity

BONUS MOVEMENT SPEED

Movement speed increased by 1.5% ⇒ Movement speed bonuses increased by 8%

REMOVEDDAMAGE CONVERSION

No longer converts bonus movement speed to adaptive force

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As a Zilean main, I considered that my 99% self-speed (which you often don't get until level 18) is one of the strongest in the entire game, so maybe I'd want to use the new rune.

However, this is not a flat 8%, but 8% of your bonus. In other words, if you use a 40% movespeed bonus (Zilean's rank 1 E), this rune only increases your movespeed by an additional....brace yourselves...3.2%. And only during the duration of that movespeed bonus (2.5 seconds.) It does absolutely nothing when you're just running around since it no longer applies to boots.

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Okay, but it gets worse. Beyond approximately 55% bonus movespeed, you hit the soft movement speed cap, which halves all bonuses thereafter. Simple example: Zilean's rank 3 E speeds you by a listed value of 70%. However, it's more like 55% + one half of the remaining 15%, i.e. 62.5% total.

What does this mean?

It means that for anyone who can use the new Celerity "decently" at all, i.e. anyone with a high self-movespeed bonus (Rammus, Hecarim, Udyr), you're actually getting HALF the listed value, i.e. a 4% bonus.

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Well I tested this myself, and it seems to hit a second cap, because it's even worse than a 4% bonus. I went into the Practice Tool and leveled Zilean's E to rank 5. I tested the difference between speeding myself around with and without Celerity.

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With Celerity: 595 movement speed.

Without Celerity: 580 movement speed.

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Folks, that's an increase of 2.58%. Only for 2.5 seconds, and on one of the strongest movement speed bonuses in the entire game, often unavailable until level 18.