To get back on topic, Sancho is plenty quick enough plus his mind and feet are quicker than any defender he will be facing. We have raw pace in Rashford and James from the bench, what we need is a creative player who can commit defenders 1 on 1 and link up play. What impresses me most about Sancho is the speed and timing in which he passes in the final third, he often catches defenses by surprise with how quickly and accurately he releases the ball and is always a step ahead of the defenders he faces. It's a unique skill which when combined with his dribbling and consistent technique makes for a perfect foil for our other attackers in my opinion.
There's been plenty of counter attacking goals for Dortmund where Sancho has travelled 60+ yards, he definitely never slows the play down and is very direct in a counter attacking position (if anything he isn't direct enough when playing against a low block) and when he skins a defender he is rarely ever caught up with. Any worries about his pace are silly really, a front 3 of Rashford, Martial and Sancho will have every defence worried about holding a high line.