Why does my Mac take so long to boot?
Have you upgraded your hard drive only to find out that the new drive is booting slower than the original? Or, maybe you haven’t upgraded the drive, but your Mac just seems slow to boot?
If your system drive is not setup as the Startup Disk, the Mac will take extra time at startup looking for a suitable drive to boot from. To make sure you have a startup drive specified, go to Apple Menu > System Preferences > Startup Disk and select your boot drive.
Clean up your Mac from the inside.
If you’re in the habit of shutting down your computer overnight, such as in an office environment, your Mac isn’t able to run some of its maintenance scripts, which are set to run in the wee hours of the morning. These scripts do things like clean up system logs, clear out old “junk” files, and repair/update internal databases. These actions help keep Mac OS X running its best.
Fortunately, it’s simple to run these scripts manually (as well as perform other system-level operations) using the freeware utility OnyX. Alternatively, you can just leave your computer on overnight once a month.
Repair your permissions.
Every file on your Mac has a set of permissions that determines not only which users can access which files, but also what files the operating system itself can access and modify. Over time, these permissions can get corrupted.
Keep your Mac’s permissions healthy by repairing these permissions. The simplest way is to open up Disk Utility, select your hard drive, highlight the “First Aid” tab, and click the “Repair Permissions” button. Then, let Disk Utility do its thing.
Doing this at least once a month and right after installing/updating software will help keep things running smoothly.
Edit: Changed “Startup Items” to read correctly as “Startup Disk.”