I remember reading that Bach's music was the most mathematically complex of all the classical composeurs - not sure if this is true?
I'm personally partial to Mussorgsky's 'A Night on the Bare Mountain', Berlioz's 'Symphonie fantastique' and Mozart's 'Requiem Mass'.
Hmm - bach - though fast, busy and containing lots of notes is actually very simple. My final year uni project was based on fractal analysis of Bach (and Schoenberg - interesting!).
Others did this better than I, and I've seen demonstrations where it's quite possible to reduce an entire Bach fuge to a mere 4 notes. Then run it through the reduction algorithm backwards, and get back a very similar definitely Bach sounding fuge. All using simple rules....
So yes - it sounds complex, but there is a simple symmetry irrespective of scale in the pieces.
anyways I digress...
Fast it certainly is - and harpsichord players cranking out the notes (e.g. the scarlatti family) were in some ways the worlds first thrash metal musicians.