MacBook running WAY slow all of the sudden?!

Back up your data since the symptoms you describe may indicate that your startup volume may be failing.

Boot OS X Recovery by holding ⌘ and r (two fingers) while you start your Mac.

At the Mac OS X Utilities screen, select Disk Utility. Select your startup volume (usually named "Macintosh HD") and click the Repair Disk button. Describe any errors it reports in red. If Disk Utility reports "The volume Macintosh HD appears to be OK" in green then you can be reasonably (though not completely) assured your hard disk is in good working order.

If this started happening without your having made any changes and without a warning of low disk space, then it's likely that the boot drive, or some other hardware component, is failing. Back up all data immediately, then run Apple Diagnostics or the Apple Hardware Test.

Even if the test is negative, you should make a "Genius" appointment at an Apple Store to have the machine tested more thoroughly.

The hardware test is independent of what OS is installed. You need the original discs to run it.

B McCoy wrote:

Will the recovery mode automatically wipe out OS X?

No.

Depending on the age of your MacBook Pro you may be able to run the Internet version of Apple Hardware Test. To do that hold option d while starting your Mac.

It is not likely to reveal anything of use but if it indicates anything other than "No trouble found", reply with its results.

Computers that can be upgraded to use OS X Internet Recovery

About the "Apple Hardware Test does not support this machine" message

Back up your data before you go anywhere near a Genius Bar. If they replace your hard disk or erase its contents, don't expect to get it back.

They'll probably ask you if you created a backup, but they might forget. If your answer is "no" they may not hear it, or care.