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Your question is landed on Mac category. So I assume you are using a Mac. If not, please let me know.
There are several ways to speed up the Mac. Clear the temporary memory by resetting the PRAM and NVRAM of your Mac.
Shut down your Mac
Restart and immediately hold down Command-Option-P-R keys at the same time
Hold the keys down until the computer restarts and you hear the startup sound for two or three times.
Release the keys and let the Mac bootup. This will clear PRAM memory (you will not lose any files)
The second method is, run Disk Utility. Go to Applications > Utilities folder and open Disk Utility. In Disk Utility, select your hard disk in left column (the first one in the list will be your hard disk), then Repair Permission. After that click Verify Disk and if asked to repair, click Repair Disk. If you are using 10.11 El Capitan, then click First Air button and do it.
Don't buy any cleaning applications like MacKeeper, CleanMyMac etc. They does more harm than any good.
If you want to clean unwanted things with third party applications, then use Onyx (free) --> http://www.macupdate.com/app/mac/11582/onyx
For AntiVirus, use this application (free) --> https://www.malwarebytes.org/antimalware/mac/
Always delete unwanted files from your Mac and empty the trash. Don't put too may files on desktop. It will slow down the Finder performance.
If you have any questions or would like any additional information, please don't hesitate to ask.