ARS Technica discusses the boot optimizations in Ubuntu 9.10, one of the interesting things is that Canonical has dramatically improved boot performance on solid state disks. The site received a boot chart that shows Ubuntu 9.10 alpha 6 booting in only five seconds on a system with an SSD:
Canonical external developer relations coordinator Jorge Castro (a former Ars Linux contributor) supplied us with a boot chart from his computer, which has an SSD and boots Karmic alpha 6 in only five seconds. If you examine his chart, you will see that Xorg comes up in only two seconds.
One of the most significant technical factors contributing to awesome SSD boot performance is the inclusion of sreadahead, a system service that uses prefetching to load data that is used by the boot process before it is needed. It will also cache the prefetched data and store it so that it can be used during subsequent boots, but it's less effective on conventional hard disks where seek latency introduces some challenges.