Many people report a slightly longer boot time, but that sounds excessive. Is it pausing for a long time(say, 30 seconds?) at the blue screen? That might indicate that something on your system is delaying the boot. Run the Console app, and review it right after a boot to see if any log entries might provide a clue. Or you could do an Archive & Install.
Your other options are to reboot a couple of times, reboot once in Safe Mode by holding Shift, wait for 10.5.2, or just wait in general and see if the system speeds up as it updates the hotfiles for the new operating system. Leave the system on for 2.5 days, sleeping allowed, and then reboot.
Make sure you've got at least a few gigs of free disc space.