The Soku Soku no Mi is a Paramecia-type Devil Fruit that allows the user to move at incredible speeds, thus making the user a Fast Human (速い人間,Hayai Ningen?).
The major strength of this fruit is that it grants the user the ability to move at incredible speeds and somewhat immune to blunt attacks. He is also able to withstand falls that would normally kill a man as long as the user falls on his feet. The user is also granted the ability to see objects moving at great speeds, even those moving at speeds greater than the user.
It also seems the user can overcome the weakness of water by simply running across it and are even able to slide across the water if they can gain speed up again to continue. It also seems that this fruit gives the user an immense appetite, so it does require enormous amounts of energy to run at the speed the user needs to.
The user also seemed to be able to create afterimages by running at great speeds, confusing the enemies. The user also is given the ability to move themselves at immense speeds to create a "phase" state where they cannot be hit by any object, as the object will move through them. Though as stated by Swift, using this ability makes him incredibly hungry so it is assumed it takes a lot of energy.