When talking about the type of speed that a footballer needs, Maguire certainly is slow. Once he actually gets up to full speed he's 'ok' (nothing more than that), but he's very slow off the mark and takes ages to get up to that top speed when compared to the vast majority of footballers. Generally players of his size will be slightly slower over the first couple of steps, but not to the extent Maguire is. Ideally they'd be like Smalling where he could be slightly slow when having to turn and take the first step or two, but then quickly got to a top speed that allowed him to make up that ground the majority of times.
De Ligt is somewhere in between, where he seems below average both in terms of explosiveness off the mark and acceleration, but obviously faster at both than Maguire. Actual top speed might be similar though.