I think we will have to agree to disagree about the movie being slow, I think it is perfectly paced in terms of the size and span its story and concepts as well as the ideas it is trying to explore.
However this is clearly a problem of purely artistic film making versus the more popcorn end of the market, so when you use the term "slow" I interpret that as you wanting to see more action and I'm afraid not all movies are made with that purpose in mind.
2001 is a film made with a deeper purpose in mind than just action, it is made to make the viewer think on an all-together deeper level about the world around them and the nature of existence itself, for that you need a more thoughtfully paced film and that is why whilst you might think it has a slow pace, I think the pace is perfect to allow the viewer ample time to consider some very profound ideas and to be able to take in the overall scope of those ideas.
Of course it in not necessary for a movie to have such a thoughtful pace in order to have meaning and to be ambiguous in some areas, however it is necessary if it is dealing with the kind of deep questions that 2001 is attempting to deal with.