These stats are always total crap. The only way for them to measure it is to find instances of highest speed during games, with no regard for context etc. Players rarely run far enough in a straight line to hit top speeds, and not every run is going to be measured, etc etc. Plus the combination of other qualities with pace is much more important than exactly how fast a fast player is. Shaw, for example, probably isn't one of the very fastest guys in the PL, but he runs with a power that makes his pace far more useful than that of, say, Erik Lamela.
We do lack pace to a degree, but it's to do with the right sort of fast players in the right positions to execute our strategy, not 'how many United players are in the fastest players stats?' We have some fast players; Smalling must be one of the league's fastest CBs, Martial is fast, Shaw is pacy. Meanwhile, the fact that Memphis isn't particularly fast and Mata downright sluggish is causing us problems at times.
This is one of those where the evidence of your eyes is much more useful than stats.
