The story does unfold slowly, but it was far from boring (in my opinion). I'm not sure why they started off with backstory and explaining why the two main characters were at the apartment block. It didn't help the story.
I thought the story was well presented and from a Sci-Fi perspective was plausible. The production values were very high with excellent special effects. The feel of the movie was very real and you felt what the characters would be feeling... so I guess this means the acting was believable.
At no time did I need to suspend my disbelief. To me, that is the mark of a good Sci-Fi movie.