Short answer: No, there isn't.
Long answer: Freespace isn't the kind of game to explain all the little, insignificant details, it's a game that's meant to be played and enjoyed. Thus, to increase enjoyment, they toned down the speed signifcantly. I honestly wouldn't be surprised if most space sims actually run at relativily the same speed (just changing the units tags to make things seem faster than they really are), so it's probably not out of the ordinary except that you actually notice it.
If you want an in universe answer, I guess it might have something to do with A) ECM and B) subspace. Wouldn't want to try to figure out the details beyond that as it would require math.