Fix iMac slow boot up times

Apart from the obvious of removing unwanted startup items, try these following fixes:
Reset PRAM and NVRAM

• Shut down Mac
• Turn on Mac
• Hold down CMD, ALT, P and R keys - hold keys down until Mac restarts and you hear the chime
• Let go of keys

Change some permissions

• Open a terminal and type

sudo chown root:wheel/

Boot in safe mode to force disk check and clear problem caches

• Shut down the Mac
• Hold down the shift key and turn the Mac on
• Wait until the machine boots
• Log in
• Shut down the Mac and start normally

If you still have trouble give your Mac a good lean using Onyx
http://download.cnet.com/OnyX/3000-18512_4-10653022.html