Its not the pace that's really important, its beating a player.
Peps management is all about creating numerical advantages, whether that's having a ball playing keeper who forces the opposition to press him, thus taking an outfield player put of the game, or a ball carrying centre back getting past the first line of defence, or a winger beating someone out wide.
The whole strategy is founded on the idea that if you take a defender out of the game then someone is free further up the pitch.
So your winger beats the fullback and runs at the CB, that means there's only 2 free defenders left and if you get 3 men in the box someone has a tap in.
So Grealish doesn't have the pace of Henry, or Sané but he has the ability to beat a player, run at the next one and then play the pass to the open man.
That's why Foden and Grealish and Bernardo have all played as wingers for us.
Sterlings success under Pep is based on the same thing except he beats players without the ball. Hes incredibly good at losing markers, drifting into space and being that unmarked man at the end of the move.