I believe this problem has existed since I upgraded to 13.04. I have an SSD, so differences in boot/shutdown times are very noticeable. It used to take a couple of seconds to shut down - now, it takes several more. I can press ESC and view the shutdown procedures, which seems to hang a long time on 'Asking remaining processes to terminate' before failing and killing them. Now it takes several more seconds.
Is there any way I can fix this problem, or at least find out which process(es) is/are refusing to terminate?