Overclock

Overclock (オーバークロック) is the Quirk possessed previously by the former hero O'Clock, until it was stolen from him by All For One. All For One eventually gave this Quirk to the villain Number 6.

Description
Overclock seemingly allows the user to accelerate to extreme speeds, to the point that to the user, it looks like nothing is moving. However, according to Knuckleduster, its true ability consists of stimulating the brain, speeding up the user's perception and thoughts relative to everything else, allowing them to move at a much faster speed than others.

In normal situations, the user can move at speeds approximately three to ten times faster than normal. In emergency situations, when the user's fight or flight response is active, the user can move at speeds a few dozen times faster than normal, and can even move so fast that the world seems completely frozen.

Overclock also grants the user heightened senses, allowing them to see, hear, and think at hyper speed. They're able to clearly hear words that have been sped up to three-hundred times the normal speed when the Quirk is active.

Combining it with his healing factor, the villain Number 6 use Overclock to promote cell growth, allowing the user to heal grievous injuries. As he has parasitic cells in his body, he can also accelerate their growth as well, allowing them to merge with parts of his body, if he so wishes, even being able to grow a new limb out of the merged cells.

The heightened senses granted from Overlock can give the user the ability to read a person's subconscious reactions to both manipulate and react to a person's reactions without the subject even knowing.

Weaknesses
Overclock has several drawbacks. It has a cool-down period between uses, and can only be used for a few seconds at most. If the user is knocked into the air, the acceleration aspect of the Quirk becomes useless.

Knuckleduster states that access to the Quirk can be sealed off if the user's brain is deprived of oxygen, and if the user has a serious brain injury, such as a concussion. If the user is intensely focused on something, the user experiences a sort of "brain drain", which reduces the amount of time they can actively use the Quirk. If the user remains too focused while using their Quirk, they can also risk depriving their brain of oxygen, sealing off access to the Quirk. Knuckleduster also implies that extensive physical injury, not just on the head, would be enough to seal access to the Quirk.