MacBook Pro suddenly slows down
If you noticed a sudden slowdown then that points to an event of some sort that is likely pointed to an issue with the HDD (IMHO). Mac OS is very sensitive to issues with the speed of the disk, which is why it benefits so much from upgrading to an SSD. My recommendations?


Back up your vital data now.
Boot the system into recovery mode, run disk utility to repair the internal drive.


I'm betting that there has been some kind of disk error. The question remains whether or not it is a recoverable disk error. Unfortunately Disk Utility is not a comprehensive disk repair tool. There are others out there (my go to is DiskWarrior, but there are others) that can fix things that Disk Utility can't. If disk utility cant repair it you can try another utility or just replace the drive.