IMac is very slow after switching to OSX Mavericks

Question

I'm still getting used to the philosophy of this site, so excuse me if I stray.  I had posted in a different thread and was asked to start a new thread.
I have a 20-inch, Early 2008 iMac and I've been upgraded to 10.9.4 for a few weeks.  I did have the fan problem, but sorted that out, forget what I did but may be able to retrace my steps.
But, the machine is so slow now - really not tolerable.  Every program I launch just takes a lot of time.  Even launching Textedit that used to just flash up is sow.  Safari is now very slow to launch.  Google, which I use all the time and was normally slow, is now excruciating.  Logging on, I have to wait a long time to even put in the PW.  Once open, most applications are tolerable.  But, like others have mentioned, the machine is just slower at everything.  I watch "Activity Monitor" to see what's using what for memory and CPU, and nothing new with that compared to 10.6.8 except it's not as good giving me the information I want in an quickly readable form.
Do I need to just go back to 10.6.8, or is this Mac's hardware is not compatible with 10.9.4?  It seemed that the upgrade was recommended for this machine.
I'd appreciate any thoughts, recommendation, suggestions. Thanks!
PS:  I was asked to send top 5 CPU and then the average User CPU....
Firefox                    6.7%
Activity Monitor     1.0 %
Window Server     0.8%
Firefox Plugin         1.1%
kernel_task          0.4%
User:                    4 - 11%
As you know, these constantly change, but I just jotted things down in the list above.  Nothing was taking any serious time.  Firefox will momentarily go up to 30%.  I know when Time Machine is slowing things down because I can see it backing up with the icon on the menu bar, but that's not only when it's slow.  It's just always slow starting applications or opening new windows in Firefox, opening a new window in Word or Excel.  I always have Activity Monitor open to watch for anything unusual, like if Firefox is bogging me down.
The question is should I expect this OS to be bogging this machine down, in general?  Thanks!

Answer 1

It would be nice to know how much RAM is installed. A report of this would help:
You should use Activity Monitor to check for:
Open Activity Monitor in the Utilities folder.  Select All Processes from the View menu.  Click on the CPU tab in the toolbar. Click twice on the CPU% column header to display in descending order.  If you find a process using a large amount of CPU time (>=70,) then select the process and click on the Quit icon in the toolbar.  Click on the Force Quit button to kill the process.  See if that helps.  Be sure to note the name of the runaway process so you can track down the cause of the problem.
Try these in order testing your system after each to see if it's back to normal:
1. a. Resetting your Mac's PRAM and NVRAM
    b. Intel-based Macs: Resetting the System Management Controller (SMC)
2. Restart the computer in Safe Mode, then restart again, normally. If this doesn't help, then:
     Boot to the Recovery HD: Restart the computer and after the chime press and hold down the
     COMMAND and R keys until the Utilities menu screen appears. Alternatively, restart the
     computer and after the chime press and hold down the OPTION key until the boot manager
     screen appears. Select the Recovery HD and click on the downward pointing arrow button.
3. Repair the Hard Drive and Permissions: Upon startup select Disk Utility from the Utilities menu. Repair the Hard Drive and Permissions as follows.
When the recovery menu appears select Disk Utility. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list.  In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive.  If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit DU and return to the main menu. Select Restart from the Apple menu.
     Reinstall the 10.9.4 update: OS X Mavericks 10.9.4 Update (Combo).
4. Reinstall Lion/Mountain Lion, Mavericks: Reboot from the Recovery HD. Select Reinstall Lion/Mountain Lion, Mavericks from the Utilities menu, and click on the Continue button.
Note: You will need an active Internet connection. I suggest using Ethernet if possible because it is three times faster than wireless.
     Reinstall the 10.9.4 update: OS X Mavericks 10.9.4 Update (Combo).
Download and install EtreCheck 1.9.12. Run on your system and copy and post results here.