Well actually alot of it is due to the lightsaber blades. This is when they were still trying to do the effect in camera. The rotating wooden blades kept breaking, so they were told to just try and stop before the blades hit each other, limiting the kinds of moves they were able to do and ultimately making it feel less intense than it could have been in comparison to the other films. It doesn't really have to do with it being slow, but a little more spread out and exciting. Even the ROTJ duel, which was confined to one room, moved around alot and was a pretty intense battle. The suggestion was simply to maybe allow for some more varied attacks (so it doesn't feel so much like they're just tapping their sabers together gently every so often) and move it out of that small area.