Well, all preferences have to be loaded in, right? menu bar items, any custom extensions you're running, like drivers, etc.
Is your computer completely froze while this is happening? Beachballing, or just generally slow?
A huge number of things could be happening. Loading in what I've mentioned. Spotlight catching up on its indexing. Time Machine doing a check. Networks being checked. Remote drives being checked. Notifications being checked. iCloud logging in and updating. RAM contents might be being loaded in from previous shutdown. Caches might be being emptied.
All sorts of housekeeping processes could be running.
My computer is kinda slow on restart, as it checks resume states and stuff (even off, it has to check to know it's off), I see menu bar items pop up one at a time as it obviously loads them etc.
I don't restart often, so it's not a big deal to me.