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Please follow these instructions :
1-Close Applications that you don't use :
Look for the applications marked with a dot on your dock, right-click or Ctrl-click their icons, and select “Quit.” :
2-Prune Startup Programs
If your Mac is slow after you log in, it may have too many startup programs.
open the System Preferences window by clicking the Apple menu icon and selecting System Preferences Click the Users & groups icon, select your current user account, and click Login Items. Uncheck any applications you don’t want starting when you log in : http://cdn9.howtogeek.com/wp-content/uploads/2015/09/img_55eb77adc5b0b.png
3-Reduce Transparency and Animations :
open the System Preferences window. Click the Accessibility icon and check the Reduce transparency option to reduce transparencies.
4-Lighten Your Web Browser
Try to minimize the number of browser extensions you use and have fewer tabs open at once to save memory and CPU resources.
5-Disable FileVault Disk Encryption
FileVault disk encryption is enabled by default on Mac OS X Yosemite. This helps secure your Mac’s files if it’s ever stolen, preventing unauthorized access to them. It also prevents people from changing the password ***** your Mac and signing in without your permission.
this may cause the Mac to be very slow to boot or sign in. If this is the case, you can head to the System Preferences window, click the Security & Privacy icon, click the FileVault heading, and turn FileVault disk encryption off.
6-Speed Up the Finder
When you open the Finder window to view your files, it opens to an “All My Files” view by default. If you have a very large number of files on your Mac, this view might be slow to load, slowing FInder down every time you open a new Finder window.
You can prevent this from happening by clicking the Finder menu and selecting Preferences in Finder. Select your preferred folder under “New Finder Windows Show” — for example, you can have all Finder windows automatically open to your Downloads folder. Finder won’t load the All My Files view anymore.
7-Free Up Disk Space
freeing up disk space can also speed things up. you can delete applications, files or folders you do not need any more.
For firewall please check this link : https://support.apple.com/en-us/HT201642
Hope this help