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There are several reasons for the slow issue. Constant slowness could be a sign of failing hard drive. Corruption of system files can also cause slow issues.
There are couple of things you can try yourself which are simple to follow. Turn off your Mac by holding down Power button for 10 seconds. Then turn it back on and immediately hold down Command and R keys at same time for 20 seconds and this will boot your Mac to the Mac OS X Utilities screen. From there, click Disk Utility. Select your hard disk at left column (Macintosh HD or hard disk name). Then click Verify Disk (under First Aid). If asked to repair, click Repair Disk. Then click Repair Permission.
If you are using latest Mac OS X El Capitan, then you will not see Repair Disk option in Disk Utility. Instead click First Aid button and run it. Repair if asked.
Then restart from Apple menu. If that didn't work, then turn it off. Turn it back on and immediately hold down Command, Option, P and R keys at the same time for 5 seconds, then release the keys and immediately hold down Shift key for 20 seconds. Then just restart your Mac.
Please let me know if that didn't work, so we can continue troubleshooting.
If all the above steps doesn't improve the performance of your Mac, then you can reinstall your OS X.
Reinstalling OS X won't delete any of your files. It will just replace the corrupted system files with new one and that should fix the problem unless the hard drive itself is failing
Restart your Mac and immediately hold down Command and R keys at the same time for 15 seconds and this will boot your Mac to the Mac OS X Utilities screen. From there, click Reinstall Mac OS X. Then follow the instructions to reinstall your Mac OS X.
If that didn't work, then restart your Mac and immediately hold down Command, Option and R keys at the same time for 15 seconds and this will boot your Mac to installation screen. Follow the onscreen instructions to reinstall.
I would recommend to backup all your important files to an external hard drive before reinstalling the OS X.