Which iMac specifically and which Mac OS X version exactly are you using?
Since when does the slow startup occur?
How much free HDD space is left out of how much HDD space?
When did you get the Mac and has the OS been re-installed since?
System preferences > startup disk > restart macintosh hd
Kamil
it will not erase your data. It will only restart your computer from HDD. Maybe it is set to boot from something else (boot camp). Just choose Macintosh HD and restart.
You should time it and post us the seconds where you first time see the Dock.
If that not helps read here:
10 seconds is bullcrap
http://www.youtube.com/watch?v=BwzDpHUerTw
http://www.youtube.com/watch?v=Sqemm1ClZIg
http://www.youtube.com/watch?v=ww36TvibnSA
It is all over 30 seconds.
I mean, not even SSD makes it under 10 seconds.
Don't worry, boot speed is the last thing you should be worrying about.
No, restarting it just restarts your startup drive. When the iMac first boots up, the time it spends in the gray screen is the time it spends looking for the right drive to boot from. Restarting the startup disk will result in your computer taking less time to look for its proper startup location.
iMacs will usually boot around 20-30 seconds. Restart the startup disk and see if it takes that amount of time!
Well 15 seconds is a decent improvement, if your earlier startup times were literally 60 seconds.
When I first got my iMac, startup times (counting from the chime) were around 25 or so seconds. I just tested my startup time from chime again, and now it's 35 seconds. I have partitioned my harddrive for windows7 and have more data written on it now, so that's probably why it's slower. Yours might be slower for same reasons as well.