His music, especially what we hear of it here, is very slow. From around the time of Bach's death composers had been working out ways of making music progress at a slower and slower pace: over a century later, Wagner and then Mahler wrote pieces that are about as slow as it is possible for music to get. -Of course, one can cheat by writing a 4/4 march and then specifying a tempo of, say, semiquaver = 1, but that tempo wouldn't be the correct tempo. Wagner and Mahler wrote music that is PROPERLY played at a snail's pace. Given that the slowness in no sense sounds too slow "snail's pace" is the wrong expression. A critic wrote of a famous Wagner conductor, "He doesn't beat time, he beats eternity." For all I know this was meant as a compliment.
I get the feeling that around the early 1970s directors worked out how to make the slowest possible films: there's "Death in Venice", and there's "Solyaris". I much prefer the former. For one thing, "Solyaris" steps over the line, or some line, and becomes soporific; "Death in Venice" is gripping from beginning to end. Not much happens, but it all happens in the right sequence, at the right pace, with photography you can get lost in
Another way of cheating with music, by the way, is to write something that doesn't really have a tempo at all. Such music sounds slow, but is really just unmusical, just as many films feel slow because they lack rhythm and form. "Death in Venice" isn't one of them. Beautiful in every respect, it will remind you of the timelessness and contextlessness of quality. You need no theoretical knowledge to respond to Visconti's mastery, as you do to respond to a lesser director's incompetence. It's a great work.