I considered that to be a byproduct of just how much faster Oberyn was. That, and the ****ing human sized sword he held in one hand. The BTS video had the fight coordinator saying it wasn't possible to have him being fast with as heavy a sword as he had. I think it's really hard to showcase Oberyn's prowess at the same time as showcasing the Mountain's. The only real remedy I can think of would have been to show a straight up fight with the Mountain and another warrior or warriors that he would have owned.
I'm assuming that the fight was one sided (in favor of Oberyn) in the book as well. I don't think that covering all the bases in this one fight would have been possible. Unless some, even just a little bit, of CGI or just throwing a large budget at the fight scene.