Step 18 of 20: Customise the Finder
By default, opening a new Finder window takes you to the All My Files view, which displays every file on your Mac. If you have an older Mac and lots of files - especially if they're image files or videos and so display a preview of their contents - this could slow down your Mac.
Change the folder that's displayed when you open a new Finder window by going to Finder Preferences, clicking the General tab and choosing a different folder from the drop down menu.
Step 19 of 20: Turn off File Vault encryption
File Vault allows you to encrypt every file you store on your Mac to keep it safe from prying eyes. It also uses lots of processor cycles, however, to encrypt and de-crypt those files. If you use it currently, switch it off and see if you notice a difference in performance.
Click on the Security & Privacy tab in System Preferences, then on the File Vault tab. Click the padlock, type in an admin password, and click Turn off File Vault.
Step 20 of 20: The nuclear option
If all else fails, and you've tried everything we've suggested without success, there is one more option: a clean re-installation of the OS. It's not a job to be undertaken lightly - you'll need to delete your entire boot drive. But, it will clear all the files that have collected in the system Library and the user Libraries over the years and which may be causing the Mac to run slowly.
I gave new life to a 2009 iMac which had been running Lion, and was grinding to a halt, by doing a clean install of El Capitan. It now feels like a new machine.
Remember to make at least one, preferably two, complete back ups of your bit drive before you start so you can copy documents, images, music and anything else you need back once you've installed the new OS.
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