Sluggish 2013 21.5" El Capitan iMac
I smell an impending death of a hard drive. For that case, I suggest an upgrade to an SSD will do. Not only you have a newer storage device but also you have a faster computer altogether.

On a side note, if an SSD isn't your fancy, you can always purchase a new HDD, but go for the better models since it will benefit you more in the long run.