Attack Speed is a weapon seal in Hyrule Warriors: Age of Calamity. It increases the speed with which attacks can be performed with the weapon.
According to this tweet by Swiffy22, there does not seem to be any forced cap on the speed increase achievable, other than the functional limitation of 6 total seal slots with a maximum 8% seal in five slots and a possible 9% hidden seal in the sixth.
Shape: Square
Can appear randomly on dropped weapons: Yes
Tiers when randomly dropped:
Tier 1 (no plus sign):
Buff when used alone: 3%
Buff when paired: 4%
Buff in a set of four: 4%
Tier 2 (single plus sign):
Buff when used alone: 4%
Buff when paired: 5%
Buff in a set of four: 5%
Tier 3 (double plus sign):
Buff when used alone: 6%
Buff when paired: 7%
Buff in a set of four: 8%
As a hidden seal:
Tier 1 (no plus sign):
Buff when used alone: 3%
Buff when paired: 4%
Buff in a set of four: 4%
Tier 2 (single plus sign):
Buff when used alone: 4%
Buff when paired: 5%
Buff in a set of four: 5%
Tier 3 (double plus sign):
Buff when used alone: 7%
Buff when paired: 8%
Buff in a set of four: 9%
Appears as a hidden seal on weapons:
Tier 1 buff:
[Boomerang] for Link: level 30 seal, tier 1 buff (3%)
[Swords of Devotion (DLC)] for [Sooga (DLC)]: level 25 seal, tier 3 buff (7%)
[Ultimate Sheikah Arms (DLC)] for [Purah and Robbie (DLC)]: level 25 seal, tier 3 buff (7%)
Note: A weapon receives a +5 damage buff from having any two weapon seals of the same suit, a +15 damage buff from having any four seals of the same suit, and a total +20 damage buff for having stacks of four seals in one suit and two seals in a different suit. Six seals of the same suit will only give a +15 damage buff. Therefore, if you wish to plan your weapon build in advance, it is important to know the hidden seals of your chosen weapon.