You need to have played the second game to understand this one. It certainly is slow, but also deliberate and detailed.
Black Mirror 3 occupies that middle strata: an admirable if middling achievement of detail, artwork, and design. Yes, it is slow, but it aspires to be a psychological, character-driven kind of horror. It is not quite Lovecraft, and some of the elements are a bit cliche; but for a game this is atmospheric material rarely used.
As for GameSpot's criticism of being bewildered to what's going on, well...I feel like the guy who's telling the other guy to see The Two Towers before The Return of the King. A movie series should not be blamed for being part of a series. Likewise, a game should not be faulted for being part of a unified trilogy. It is made for the initiated, and wastes little time to establish itself, which would only bog it down.
So if you feel like playing detailed, character driven psychological horror that's not really scary at all, give Black Mirror 3 a try. Or I should say, give Black Mirror 2 a try, and then Black Mirror 3. (for those curious, you do not need to play Black Mirror 1 to understand 2 and 3)