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Restart the computer
If you wonder "why my computer is running slow and freezes?," it is possible that a (unresponsive) piece of software running in the background and draining the computer's resources is the reason. Restarting can help if your computer gets slow, because it will close software which are running in the background (and using your computer's resources) even though they're not needed any more, or if a software has become unresponsive.
Also, rebooting should help if you haven't shut your computer down for some time and it performs tasks slowly.