Sancho's top-end speed is nothing to write home about in the world of wing play, but then, it doesn't need to be as his short-burst acceleration is top percentile and an attribute that's harder to deal with than outright pace.
Reason being is that Sancho, like a: Robben, Messi, Neymar or Hazard can bring the play and his defenders to a halt whilst in complete control of the next set of actions with said defenders having no choice but to be reactive rather than proactive the entire time. In short, closing him down is done at the defenders peril: too fast, and they sell themselves and, as we have seen, are easily taken out of the play. Too slow, and they enter his world of short, unpredictable bursts in any direction off of either foot. This leads to defenders having to elect to keeping their centre of gravity intact whilst being tentative fully aware that any commitment to one side of their body or the other will make Sancho's choice for him at a short-burst acceleration that very few can hope to entertain.
Allied to this, Sancho is one of the best short-chain and combination passers in the world, which adds another, horrible, variable for defenders as they also have to stay 'light' to contend with give and goes from standing starts that are immediately followed up by unpredictable, multi-directional bursts into either open space or creates space.
In combination with having a sharper mind with considerably shorter processing time, Sancho's attributes, before bringing either his dribbling or first touch (which he uses to immediately disrupt balance/equilibrium) are factored in mean, top-end pace will not be the biggest issue so much as sharp teammates who can play combination football with high levels of technical ability will be.
Put Sancho in the right team and he will easily occupy up to 3 men all game long. More pace is always nice, but for the type of player with the myriad of top percentile attributes he has, it won't be determinate of the player he ultimately can become. The biggest detriment to that would be injuries that ruin his mobility or short-burst acceleration.