Not sure why Gattaca is mentioned in this thread but I guess everyone has their own "sleepers"
Pacing doesn't have anything to do with boring but I guess it's easier for a slow paced movie to fall into the boring section, if the story/acting isn't good enough.
Boring to me is a movie that fail to deliver what the movie is about.
If it's supposed to be a comedy but I don't laugh, then it's a pretty boring movie. If it's supposed to be a drama and it fails to draw me into the story then again it's a boring movie. If it's a thriller or "puzzle-solving" movie and you know who the bad guy is in the first 5min (unless already revealed by the story itself) then it's not a particularly entertaining movie.
So if I take Thin Red Line as an example.
The war part (what was it 5-10min out of 3 hours ? ) was quite intensive and during that time I got drawn into the movie.
The other 2.5hr+ was just plain boring.
From what I remember they were at the beach, some guy talking all the time etc. etc.
On top of it, the 5-10min of the movie that actually was interersting I had already seen in the trailers for the movie
Again it's not that it was just 10min action, the problem was 2.5hr's of one guy spacing out at the beach