Pace undoubtedly is essential if you want to be successful though. Your whole team doesn't need to be rapid, but if you lack pace in attack then your options become very limited.
United have got players who move the ball quickly, but they often struggled to break teams down last year because they lacked pace. Nobody can doubt the speed of thought of players like Mata, Zlatan, Carrick and Rooney, but they were often just moving the ball round in circles as they lacked the pace to penetrate. They looked a better unit when Rashford came in up front - he's not as good technically but gave a different dimension to their play.
Any defender will tell you that the thing they fear most is real pace. A pacey outlet often means teams drop deeper, creating space in the midfield, allowing more freedom for creative players to flourish and relieving pressure on the defence. When a team knows that they're not facing pace they can push up as high as they like, congesting midfield and harrying defenders, knowing you have no chance of making them pay for it.
There's really no point in having a great target man to hold the ball up if the rest of your players aren't quick enough to get up and support him, he's basically just doing a defensive shift in that case rather than being an attacking focal point.