The speed of startup will be fastest with a totally "bare bones" OS...anything optional, leave out. The more core BB apps and 3rd party apps you install, the longer the boot up process will take. That's just the way it is.
So, the answer to your question is to remove all apps that you don't need, at which point your BB will boot up as fast as it can.
An even cleaner method would be to perform a clean OS reload, installing only those things you require.
Occam's Razor nearly always applies when troubleshooting technology issues!
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in addition to sdgardne's fine reply, I may add that if you do a battery pull when the File Free is low, then the bootup sequence takes more time (compared to a battery pull when there is plenty of File Free).
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