Ubuntu's always been very fast on my computer. I have 3GB RAM, and previous versions booted in under 40s and shut down in under 5s. Most recently, Natty took 40s to boot and only 4s to shut down. However, since installing Oneiric, my boot time has gone up to 50s and my shut down time to over 11s! A slight increase is fine, but this is quite alarming. I wait for applications to properly terminate, so that's not the issue. The only startup applications I have are Jupiter and Caffeine.
The problem is the links
I've reported a bug in Launchpad.
For what I know, the only version where booting speed was a factor was Ubuntu 10.10. After that the developers focused on other stuff like better integration between apps, Better Unity usage and feel, compatibility with Gnome-Shell for those users that wanted that, Changes in how apps communicated between each other and many other features that we actually take for granted.
Am not a huge fan of Unity myself but I can not deny that 11.10 came with a punch of big nice changes for Unity and how way we use a desktop. Adapting to Unity is not easy but so far I have seen nice results.
Returning to the question at hand, this also happens to me but is not such a big deal if what is between (Actually using Ubuntu) has been enhanced a lot of ways. It would not surprise me if in some of the minor updates that come every so many days, a small fix would improve the speed of boot time and shutdown time.
+1 to your question since I was also wondering the same thing but seeing the progress Ubuntu made in just 6 months makes us think about how good will 12.04 be.
I had a similar experience and disappointment after installing Ubuntu 11.10 (Oneiric) I looked some more into it to find out (in my case) this was caused by the network-manager. While shutting down it seems to time out and end with the message "nm-dispatcher.action: Caught signal 15, shutting down. ..."
I removed the network-manager and configured my ip address by using /etc/network/interfaces and it is now fast again.
Might be you have the same problem ....