Why does iOS take so long to start up?
Well - at the most basic level iOS and Mac OS X are the same kernel and startup processes using launchd and standard unix-heritage programs.

iOS has full sandboxing and arguably tighter access control infrastructure to build up - so in some ways, iOS does more to start up than Mac OS X does.

Once you add on mac things, of course it is more extensible so many people have much bigger loads to lift on an OS X reboot than an iOS boot.

I would argue that all the above arguments are moot and to a first approximation, you are seeing the CPU capability of the Mac hardware shining as well as the read speed of the mac storage as opposed to the iOS storage and the effects of more RAM being a better advantage and more than offsetting the larger library and binary size on Mac OS X.