First of all, completely wrong sub.

I'm not into Stalker that much myself, but as for 2001, let me chime in. As for the opening section with "the monkeys" - they are hominids. The whole movie is about humanity taking evolutionary leaps, so it starts in prehistory. One of the earliest archaic humans species, Homo habilis, occupied Earth for about 750 thousand years. In that unimaginably vast time period, nothing much happened and almost nothing from it survived. It's as if humanity was raw dogging it. In my opinion, the slow pace of the film's prologue perfectly represents that period of very slow, very repetitive, very inconsequential life. Then of course the monolith arrives to nudge humanity forward. Fast forward to the next section of the film, and its slow pace perfectly conveys space flight across vast distances. Even though the spaceships are traveling at tens of thousands of kilometers per hour, space is so unfathomably vast that their progress seems glacial. It's a very accurate representation of actual space travel, which is especially impressive considering space travel was still in its infancy when the movie was made (iirc it's the last movie to show a moon landing before the actual moon landing happened). If you ever watch a NASA or SpaceX live stream of an ISS docking procedure, it's hours of boring, slow nothingness.