Boot time depends on a lot of things on pc and phones. Is it a very clean stock rom without other apps then what followed with the phone?
I re-flashed and re-installed my phone so many times but it never goes very much faster if faster at all.
Its a lot of stuff needed to be started so it might just start what needed for the phone to work and then when you choose a "non-standard" app it starts new stuff. Its like your pc when it boots it dont open every software on the pc only what is needed