CT500MX500SSD1 Crucial MX500 500GB 3D NAND SATA SSD
[CT500MX500SSD1]

5 of 5 Stars My iMac was getting dreadfully slow. I was using a fraction of my ram but for some reason Safari was using the paging file on the hard disk. Researched this and found that this could because the hard drive was going bad so decided to go to a SSD. Installed the SSD, restored from time machine backup and boom. It now takes just 13 seconds to get to the lockscreen. If your HDD is 3 or more years old now, you should consider replacing it with an SSD. They are not that expensive anymore.
This is a quantum leap in performance. Highly recommended. Optimizing at-work devices being my primary job function as the IT manager at our company, this is the best SATA III SSD so far . Hands down.