I have 3 auto playlists I use for playback and syncing, using a whole bunch of stuff to prioritise higher rated tracks/albums.
These playlists are driven by around 15 auto playlists each. While it isn't that many auto playlists, they are very interdependent
I've only noticed any slowdown when trying it out on some really old hardware (I was bored...), and only when playlists are updated. Turning off the auto update option fixed that, though I want the automatic updates.