I was quite late coming to Alien, and I didn't much like my first viewing partly because I don't actually enjoy suspense. I appreciated it more on rewatching. And it seems to me that it's quite well-paced; plenty does happen.
If anything, Aliens is the slow one, at least in the original cut. Nothing happens for the first half hour. It's just Ripley trying to explain to the board how she lost her spaceship. It gets better once they wake up on their mother ship, and, well, it keeps getting better. But it's slow at first. (I don't like the Director's cut as much as the original, but some of the added scenes do help the early pacing.)
In general, the films I tend to dislike on first viewing are ones I come to after they've been massively hyped. The Matrix was another. I was mostly paying attention to the story, which is thin if you are used to SF. I still prefer the second film, but I can appreciate the first now for being more cinematic.