Vista - Vista SP1 Boots *much* slower than pre-SP1: why?

I pulled SP1 off MSDN a couple of days ago and installed it on my Vista
Ultimate x64 machine. Previously, booting took about one minute; now it
generally takes 95 seconds or more. The culprit seems to be SMSSInit --
before it was taking ten seconds or less, now it's taking over 35 seconds!

I've done dozens of reboots over several days, so prefetch and superfetch
are probably doing all they're going to by now.

So... why is it taking so long for SMSS to intialize? What can I do about
it? Why can SP1 boot *much* faster than pre-SP1. So far, I'm disappointed
enough with SP1 boots times that I'm probably going to uninstall it.

A little more info... the log now shows the "culprit," but gives no useful
information about what to do. To wit:

Session manager initialization caused a slow down in the startup process:
Name : SMSSInit
Total Time : 35017ms
Degradation Time : 14124ms
Incident Time (UTC) : 3-14-2008 2:59:37 pm

Is there a way I can get more detail about what exactly is slowing the boot
via smss init?