I upgraded my imac to Mountain Lion and my iMac has been painfully slow ever since. Any ideas what might be wrong and how to fix it?

Question

I upgraded my iMac to Mountain Lion and it has been painfully slow ever since.  Any ideas on what might be happening and how to fix it?

Answer 1

First, back up all data immediately, as your boot drive might be failing.
There are a few other possible causes of generalized slow performance that you can rule out easily.
If you have many image or video files on the Desktop with preview icons, move them to another folder.
If applicable, uncheck all boxes in the iCloud preference pane.
Disconnect all non-essential wired peripherals and remove aftermarket expansion cards, if any.
Otherwise, take the steps below when you notice the slowdown.
Step 1
Launch the Activity Monitor application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad. Click Utilities, then Activity Monitor in the icon grid.
Select the CPU tab of the Activity Monitor window.
Select All Processes from the menu in the toolbar, if not already selected.
Click the heading of the % CPU column in the process table to sort the entries by CPU usage. You may have to click it twice to get the highest value at the top. What is it, and what is the process? Also post the values for % User, % System, and % Idle at the bottom of the window.
Select the System Memory tab. What values are shown in the bottom part of the window for Page outs and Swap used?
Next, select the Disk Activity tab. Post the approximate values shown for Reads in/sec and Writes out/sec (not Reads in and Writes out.)
Step 2
If you have more than one user account, you must be logged in as an administrator to carry out this step.
Launch the Console application in the same way you launched Activity Monitor. Make sure the title of the Console window is All Messages. If it isn't, select All Messages from the SYSTEM LOG QUERIES menu on the left. If you don't see that menu, select
View ▹ Show Log List
from the menu bar.
Select the 50 or so most recent entries in the log. Copy them to the Clipboard (command-C). Paste into a reply to this message (command-V). You're looking for entries at the end of the log, not at the beginning.
When posting a log extract, be selective. Don't post more than is requested.
Please do not indiscriminately dump thousands of lines from the log into this discussion.
Important: Some personal information, such as your name, may appear in the log. Anonymize before posting. That should be easy to do if your extract is not too long.