He's definitely not as fast as a lambo but he'd leave one for dead over 10 yards, if the lambo didn't know where he was heading before he set off.
That's what it's all about in the small spaces on the pitch, where players with the ball need to create that little bit of breathing room to see where people are, to pull players in and to lay off to a companion or try to chip the keeper or whatever. That quickness of the mark and quick thinking are all it takes.
With Grealish, and players like him, it's not about 0 - 60 at breakneck speed, it's about enough initial speed to keep the ball, create space, play the ball or shoot. He doesn't need a sub 10 second 100m because he's never had that, so copes fine without it.
Regards speed; what he needs is what he's got and you only need to take time out to watch him play to see that.
Could David Silva have kept up with Walker over 100m - nah mate, could Silva have swiveled his hips and left Walker for dead over 10 yards? Yes mate, every time.