4/6 Cav/Inf ratio in stacks ranging from 12k-25k. AFAIK Army composition has never had anything to do with how fast a siege completes (not counting artillery), has this been changed at all?
Don't think so.
I'm surprised at your slow sieges though, I can't say I've noted them go any slower than the AI's, with the exception that the AI tends to assault more than I like to which obviously speeds things up.