Realistically, Celerity gives very little AP

Celerity (Sorcery tier 2): "Gain 3% increased movement speed and add 8% of your bonus movement speed to your AD or AP, adaptive."

This means in order to gain just 20 AP, the same amount that could be gained from Transcendence going only 10% extra on CDR, you would need 250 bonus movement speed,

Realistically, this isn't happening for most characters without majorly compromising a normal build with stuff like . Even combining it with Waterwalking and Phase Rush barely manages to make it past 19 AP for most users.

With that said, of course the rune is busted. 3% movement speed with any amount of AP is already crazy strong. But there seems to be a popular sentiment that Celerity is being taken because it provides the most damage over Transcendence and Absolute Focus, when it just isn't the case.

In regards to Absolute Focus, it provides 5 AP at level 1, 15 AP at level 6, and 26 AP at level 11; so all the way through the middle game, Celerity would need 63 bonus movement speed at level 1, 188 at level 6, and 325 at level 11 to outdamage absolute focus.

Celerity is not taken for its damage, and taking it hoping to exploit interactions with movement speed is unwise. Transcendence provides more damage and more open builds with items that add much greater utility if you're looking to take advantage of this rune slot for extra damage ( , any 30% cdr build). But for Celerity, take it for the movement speed it gives on its own, not for the damage added.