I don't run 5km races, the shortest I would do is 10km and I am actually slower than a run of 10 miles. I find it very difficult to set off at a fast pace right from the start. I need at least 3-4 miles to get comfortable and start running well. From then on I'm faster but it's too late. If it's a half marathon I have 9 good miles, if it's a 10km I only have 2 and that's not enough to make up the deficit and consequently the average mile time ends up slower.
I am not sure why this is, maybe I am just more naturally suited to longer distances or maybe there's a psychologiocal aspect in that I know I'm no good a short, fast races which translates to a poor performance but whatever it is, I don't mind as the Marathon is my favourite distance anyway.
I view 10km races purely as a training exercise and concentrate my serious racing efforts at the longer distances.