The second tri just flew by. The first, I felt it dragged on and on because you experience so much - disbelief and the nausea especially - and you're also overwhelmed by what it all means, the realization that there's so much to do/learn in anticipation of baby's arrival, and of course, most people feel compelled to keep it all a big secret until they reach the 12-week mark (that's really what made it feel so long!).
I've only just begun my 3rd but it doesn't feel like it's dragging (yet). There's so much happening still - baby is much more active, I'm getting used to my ever-changing body, etc.