Imac sometimes runs slow

Question

This machine is normally very fast it has a 2.8GHz Intel Core i7 processor and 8GB of 1067 MHz DDR3 memory.
Have tracked the memory usage in the Activity Monitor and it appears to be just over 25% with say mail, ical, a large excel file, a word document plus Safari all open, can't make my mind up whether it is Time Machine/External back-up drive or Mail causing the problem.
Any thoughts

Answer 1

First, back up all data immediately, as your boot drive might be failing.
Step 1
This diagnostic procedure will query the system log for messages that may indicate a hardware fault. 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. I've tested them only with the Safari web browser. If you use another browser, they may not work as described.
Triple-click anywhere in the line below on this page to select it:
syslog -k Sender kernel -k Message CReq 'Channel t|GPU D|I/O|nspace-h|n Cause: -' | tail | open -ef
Copy the selected text to the Clipboard (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).
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.
Reset the System Management Controller.
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.
Disconnect all non-essential wired peripherals and remove aftermarket expansion cards, if any.
Check your keychains in Keychain Access for excessively duplicated items.
Boot into Recovery mode, launch Disk Utility, and run Repair Disk.
If you're booting from an aftermarket SSD, see whether there's a firmware update for it.
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 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 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 (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.