The thing is, there really is no accepted "speed" for Vanilla X-Com. It all depended on how fast your computer was. Every animation in the game relied on it. If you computer was faster, the explosions were faster, and vice-versa. My guess is you had one of the faster computers.
On the other hand, I wouldn't mind if they were sped up a little bit so that they weren't a lot longer than the explosion sound