iMac running slow

Go step by step by step and test.

1. Power off the router. Unplug it from the wall. Wait a while.

Plug it back to the wall. Power the router on. Wait until all the lights are lit properly. It will take a while.

Restart the computer.

Start up in Safe Mode.

http://support.apple.com/kb/PH14204

2. Delete Caches.db

Close all windows and quit all applications.

Hold "option" key down and click "Go" menu in the Finder menu bar.

Select "Library" from the dropdown.

Library > Caches > com.apple.Safari > Caches.db

Right click the Caches.db file and select "Move To Trash.

Close windows and relaunch Safari.

3. Empty Caches

Safari > Preference > Advanced

Checkmark the box for "Show Develop menu in menu bar".

Develop menu will appear in the Safari menu bar.

Click Develop and select "Empty Caches" from the dropdown.

4. Delete Cookies

Safari > Preferences > Privacy > Cookies and other website data:

Click “Remove All Website Data”.

5. Safari > Preferences > Extensions

Turn off Extensions if any and launch Safari again to test.

Turn on those one by one and test.

Can you run EtreCheck and post the report here please?

EtreCheck: https://discussions.apple.com/docs/DOC-6173

How often do you reboot your Mac? There's a topic where users running Yosemite have reported that leaving their Mac on for extended lengths of time caused a very noticeable slowdown in system and application performance. It was reported that those rebooting frequently experienced no such slowdown. I prefer to shut down each night and never seen any performance hits under Yosemite.

Onyx doesn't defrag. It runs maintenance scripts and empties various caches. It's not really needed. Have you tried booting into the Recovery volume (boot with the Command + R keys held down), selecting Disk Utility and repairing the disk permissions and the disk?

Also download and run Etrecheck as dominic23 has suggested. Copy and paste the results into your reply. Etrecheck a diagnostic tool that was developed by one of the most respected users here (and recommended by Apple Support personnel) in the ASC to help identify the more obvious culprits.

Add to the list - check your top sites and any RSS feeds you have -- if you are not using top sites - check anyway - tops sites add themselves!!!

Top sites and RSS feeds are refreshed when you open Safari - that means they go out and get the latest version of the page/RSS.

These add to cache and cookies.

Saw the same topic --- I also have always shut the mac down when no in use.

Shutting down - does clean-up that reboot and sleep do -not - some companies will on their big blue computers - schedule a power down/restart once a week to do the exact same thing.

It would help us to help you if we could have some more technical info about your iMac.

If you like, please go ahead and download, install and run Etrecheck.

Etrecheck was developed as a simple Mac diagnostic report tool by a regular Apple Support forum user and technical support contributor named Etresoft.

Etrecheck is a small, unobstrusive app that compiles a static snapshot of the current state of your entire Mac hardware system and installed software.

This is a free app that has been honestly created to provided help in diagnosing issues with Macs running newer versions of OS X.

It is not malware and can be safely downloaded and installed onto your Mac.

http://www.etresoft.com/etrecheck

Copy/paste and post its report here in another reply thread so that we have a complete profile of your Mac's hardware and installed software so we can all help with your Mac performance issues.

Thank You.

It wouldn't hurt. It's another area to eliminate the possibility of troubles or issues.

And, I don't believe anyone has asked:

How much RAM do you have? There is a general recommendation that 4 GB should be considered an absolute minimum - more is better depending on your workflow.

Cookies

You can delete all cookies except the ones from Apple,your ISP and from your banks.

Best.