I don't think booting in N900 is slow comparing to some other phones .. try to completely shut down an iPhone or blackberry then turn it on again .. it will take longer time than N900 to boot up .. and if we look at the operation system in iPhone and blackberry they will be lighter than maemo on N900 , cause iPhone and blackberry are just smart phones but maemo N900 are much like PC as u said . so , N900 with this powerfull operation system is fast booting comparing to other phones with lighter operations .
And if want to compare booting speed in N900 to older Nokia devices like N95 or older , u are make a mistake, because the operation systems in those old phone are very simple and just nothing comparing to operation in N900 . so it's not strange if they are faster in booting ..
anyway , u can speed up booting by delete video booting and disable some features that must work when phone starts up , such as : wireless , chat accounts , use less number of desktops specially if u have 9 desktops , 2 will be enough.. also if u have less apps and more rootfs space , the boot will be faster .. I'm not professional , but I think u will get faster booting if did that .