Q: Mac being very slow since Yosemite update. HELP

When you see a beachball cursor or the slowness is especially bad, note the exact time: hour, minute, second.  

These instructions must be carried out as an administrator. If you have only one user account, you are the administrator.

Launch the Console 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 and start typing the name.

The title of the Console window should be All Messages. If it isn't, select

          SYSTEM LOG QUERIES ▹ All Messages

from the log list on the left. If you don't see that list, select

          View ▹ Show Log List

from the menu bar at the top of the screen.

Each message in the log begins with the date and time when it was entered. Scroll back to the time you noted above.

Select the messages entered from then until the end of the episode, or until they start to repeat, whichever comes first.

Copy the messages to the Clipboard by pressing the key combination command-C. Paste into a reply to this message by pressing command-V.

The log contains a vast amount of information, almost all of it useless for solving any particular problem. When posting a log extract, be selective. A few dozen lines are almost always more than enough.

Please don't indiscriminately dump thousands of lines from the log into this discussion.

Please don't post screenshots of log messages—post the text.

Some private information, such as your name, may appear in the log. Anonymize before posting.

How long ago did you either activate or deactivate FileVault?

If you don't already have a current backup of all data, back up before proceeding. There are ways to back up a computer that isn't fully functional. Ask if you need guidance.

Start up in Recovery mode. When the OS X Utilities screen appears, select Disk Utility.

In the Disk Utility window, select the icon of the startup volume from the list on the left. It will be nested below another disk icon, usually with the same name. Click the Unlock button in the toolbar. When prompted, enter the login password of a user authorized to unlock the volume, or the alternate decryption key that was generated when you activated FileVault.

Then, from the menu bar, select

          File ▹ Turn Off Encryption

Enter the password again.

You can then restart as usual, if the system is working. Decryption will be completed in the background. It may take several hours, and during that time performance will be reduced.

If you can't turn off encryption in Disk Utility because the menu item is grayed out, you'll have to erase the volume and then restore the data from a backup. Select the Erase tab, and then select

          Mac OS Extended (Journaled)

from the Format menu.

You can then quit to be returned to the main Recovery screen. Follow these instructions if you back up with Time Machine. If you use other backup software, follow its developer's instructions.

Don't erase the volume unless you have at least two complete, independent backups. One is not enough to be safe.

That's right.

Is there a way of just starting from scratch and do a factory reset?

If you haven't already done so, try this simple step first.

Back up all data to at least two different storage devices, if you haven't already done so. One backup is not enough to be safe. The backups can be made with Time Machine or with Disk Utility. Preferably both.

Erase and install OS X. This operation will destroy all data on the startup volume, so you had be better be sure of the backups. If you upgraded from an older version of OS X, you'll need the Apple ID and password that you used, so make a note of those before you begin.

When you restart, you'll be prompted to go through the initial setup process in Setup Assistant. That’s when you transfer the data from a backup.

Select only users in the Setup Assistant dialog—not Applications,  Computer & Network Settings, or Other files and folders. Don't transfer the Guest account, if it was enabled.

After that, check the App Store for software updates.

If the problem is resolved after the clean installation, reinstall third-party software selectively. I can only suggest general guidelines. Self-contained applications that install into the Applications folder by drag-and-drop or download from the App Store are usually safe. Anything that comes packaged as an installer or that prompts for an administrator password is suspect, and you must test thoroughly after reinstalling each such item to make sure you haven't restored the problem.

I strongly recommend that you never reinstall commercial "security" products or "utilities," nor any software that changes the user interface or the behavior of built-in applications such as Safari. If you do that, the problem is likely to recur.

Any system modifications that you do choose to install must be kept up to date. None is required for normal operation.

Before installing any software, ask yourself the question: "Am I sure I know how to uninstall this without having to wipe the volume again?" If the answer is "no," stop.

Never install any third-party software unless you know how to uninstall it.