Well, in the third season there was more action so perhaps this is what you perceive as more exciting. But there were also fairly lame episodes in it like the one with the Wild West planet or where some crewmembers changed into an extinct species.
I think the modern Trek shows are all told fairly quickly. TNG had a couple of minutes more than post-TNG and TOS about 10 minutes more and I wouldn't say that they dragged because of it. ENT did encounter some problems though with its three-parters in the last season, one episode always felt a bit superfluous, but then again this is unavoidable when you do more serialized kind of story-telling.