Good advice above but I wonder...
Since the slow down occurred did you update or upgrade Mac OS? When I updated from Lion to Mountain Lion my boot time increased loads. I basically upgraded over the top of the previous installation which I guess is what caused it. When I cleanly reinstalled Mountain Lion (having made a recovery disk (bought Mtn Lion from Appstore), my zippy boot times were back to normal.
One thing I did to get a really fast boot time was to install a hybrid hard drive, a
Seagate Momentous XT. These are laptop hard drives but no reason why it shouldn't work for your iMac I don't think. It has a Solid State Disk (SSD) partition in it in which it stores commonly used code including OS and boot up stuff, and learns which things you use most often so you get a speed boost after using it a little while. If you already have an SSD installed obviously it makes no sense to get the hybrid drive.
If you don't wanna mess with things too much then a clean install is probably the best solution. I don't know but can the Time Machine back up thing slow down booting? Maybe disconnect it and see? Have as little connected as possible too (if applicable) just to time the start up. Use your stopwatch phone app mate.
If you've got the space for a larger 3.5" (desktop size) drive, you could maybe
go for this one which I just discovered this second.
Edited by deleted (Sat 28-Sep-13 02:27:52)