I have had the slow boot up in my Mavericks install in the past and have, at times, boiled it down to connected devices. On these occasions I have done the usual things, zapped the PRAM, SMC reset, fixed Permissions, etc. with little or no effect.
Just the other day I had the slow boot up again. It began when I restarted into Mavericks from my Yosemite Beta install. It took forever to boot into Mavericks, but it finally completed. I tried all the usual procedures listed above and a few other things.
Then I remembered on previous occasions the slow startup was related to external devices. After a little trial and error I discovered my external Time Machine backup drive was stuck. It was frozen, nothing was happening.
Here is what I did. I left the Time Machine drive plugged into my iMac, but I turned it off at the drive. I waited a few seconds then turned it back on. It seemed to be functioning properly. A restart proved me correct. The iMac restarted in about 16-17 seconds which is normal.
This is what I think happened. When the iMac was starting up it searched for connected devices. I think it saw the Time Machine drive connected but could not actually acknowledge it was connected during the boot process because the Time Machine drive was frozen. After trying for a while, it would give up, bypass the drive and finish startup. Once I reset the Time Machine drive, the iMac went through it’s normal checklist during boot up and started up within a normal time frame.
So, the moral to the story is, if your are having slow startup times on your Mac, by all means do all the tech stuff first (zap the PRAM, SMC reset, repair Permissions, etc), but do not forget to check your connected devices as a last resort. Try disconnecting them or, if it is an external HD, just turn it off and and back on again and see if that resolves your issue.