IMac is sluggish

Question

My iMac is sluggish and stalling, needs to be rebooted often? This just happened suddenly last week and now it happens everyday.

Answer 1

Back up all data immediately as your boot drive may be failing.
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 'Chann|GPU D|I/O' | 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 window will be empty. If you get output like this:
kernel[0] <Debug>: disk0s2: I/O error
the boot drive is failing, or there's some other hardware fault in the storage subsystem.
If you get output like any of this:
kernel[0] <Debug>: ** GPU Debug Info Start **
kernel[0] <Debug>: ** GPU Debug Info End **
kernel[0] <Debug>: NVDA(Private): Channel timeout!
kernel[0] <Debug>: NVDA(OpenGL): Channel timeout!
there's a fault in the graphics processor or logic board. The two kinds of output are not mutually exclusive, but if you get both, a logic-board fault is likely.