I ran Windows Update using IE8 a few days ago using both the Express and Custom
options. Both took very close to 10 minutes, so that is consistent with your first
test.
Interesting that your second test on a different machine took only two minutes.
I wish I had not even clicked on the Custom option. It offered an update to my
AC97 Audio Driver and an update to my Motherboard Chipset Drivers. After restarting
the system, I had no audio device at all. I had set a Restore Point and used System
Restore to reverse the update.