My iMac runs slow-how do I clean it up

Question

my iMac runs slow-how do I clean it up

Answer 1

First, back up all data immediately unless you already have a current backup. If you can't back up, stop here. Do not take any of the steps below.
Step 1
This diagnostic procedure will query the log for messages that may indicate a system issue. It changes nothing, and therefore will not, in itself, solve your problem.
If you have more than one user account, these instructions must be carried out as an administrator.
Triple-click anywhere in the line below on this page to select it:
syslog -k Sender kernel -k Message CReq 'GPU |hfs: Ru|I/O e|find tok|n Cause: -|NVDA\(|pagin|timed? ?o' | tail | awk '/:/{$4=""; print}' | open -ef
Copy the selected text to the Clipboard by pressing the key combination command-C.
Launch the Terminal application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad. Click Utilities, then Terminal in the icon grid.
Paste into the Terminal window (command-V). I've tested these instructions only with the Safari web browser. If you use another browser, you may have to press the return key.
The command may take a noticeable amount of time to run. Wait for a new line ending in a dollar sign (“$”) to appear.
A TextEdit window will open with the output of the command. Normally the command will produce no output, and the window will be empty. If the TextEdit window (not the Terminal window) has anything in it, stop here and post it — the text, please, not a screenshot. The title of the TextEdit window doesn't matter, and you don't need to post that.
Step 2
There are a few other possible causes of generalized slow performance that you can rule out easily.
Disconnect all non-essential wired peripherals and remove aftermarket expansion cards, if any.
Reset the System Management Controller.
Run Software Update. If there's a firmware update, install it.
If you're booting from an aftermarket SSD, see whether there's a firmware update for it.
If you have a portable computer, check the cycle count of the battery. It may be due for replacement.
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. See whether there's any change.
Check your keychains in Keychain Access for excessively duplicated items.
Boot into Recovery mode, launch Disk Utility, and run Repair Disk.
If you have a MacBook Pro with dual graphics, disable automatic graphics switching in the Energy Saverpreference pane for better performance at the cost of shorter battery life.
Step 3
When you notice the problem, 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 View menu or 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 Memory tab. What value is shown in the bottom part of the window for Swap used?
Next, select the Disk tab. Post the approximate values shown for Reads in/sec and Writes out/sec (not Reads in andWrites out.)
Step 4
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 by pressing the key combinationcommand-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.