WHy did my MacBook Pro become so slow all of a sudden?

There are quite a few different issues that could explain it. I would start off clearing out your old cache & log files. Here's a good tool for that: Disk Doctor did that help?

• Did you upgrade your drive?
• Did you put in a SSD drive?

If either of these are true you may have a SATA I/O speed issue as your system can only support a SATA II (3.0 Gb/s) drive. Make sure the drive you put in is able to run at this slower speed. Review the drives spec sheet. As an example here is a Seagate 2.5" SSHD spec sheet. Note the line Interface this drive offers auto speed sense technology so it will match up with the systems SATA port I/O speed. Not all drives (HD or SSD) do this! If it is not able to run at SATA II you'll need to get a different drive. I also recommend you replace the drive cable following this IFIXIT guide: MacBook Pro 13" Unibody Mid 2010 Hard Drive Cable Replacement if you have banged your system a bit as the cable can become damaged. And lastly, make sure your systems firmware is current. Here is an Apple TN on how to check: About EFI and SMC firmware updates for Intel-based Mac computers you want something at or newer than whats listed here.

• Is your HD very full (do you have 1/4 of it still free)?
• Have you recently upgraded your OS or applied updates?

If either of these is true you likely need to repair & defragment your hard drive (SSD's do need the free space and don't need defrag'ing. They still need a bit of TLC so do run Disk Utility). Make a full backup of your stuff, then create a bootable external drive as you'll need it. Using it to boot your system run Disk Utility from it to check your drive do you have any errors? If not go to the next step in getting a good defragmentation app like this one: Drive Genius. That should fix things. If your drive did come back with errors you may have a drive thats dyeing or has some bad corruption. Reformat the drive, install a fresh copy of OS and then restore your backup back on if the drive test out OK. Otherwise its time for a new drive. I do recommend going with a hybrid drive like the one I posted above if you don't want to go to a SSD. While SSD's are fast they are still quite costly when you want deep storage, a hybrid drive is a good half step.