Another option is as below :-
1. Backup all ur data & apps
2.Install OS on PC & Shrink OS. Remove most of junk & select only core apps
3. Do a clean wipe with BBSAK
4. Do a fresh install with only core apps.
5. See Bootup time. If still it is low, try with a diff OS -preferable old OS with which the handset was launched.
6. If that too gives, slow boot chk ur device memory etc. It could be buggy!!!
Yeah, I did shrink my OS before I installed it. I took out all the apps that need a data plan, like BBM, Twitter, MSN, all of those. I guess I'll have to update. I'll do it when school ends, though