If you have a Samsung 840 EVO SSD, you may have noticed a loss in performance if you've owned it for a long while. Unfortunately, a bug in the design of the 840 EVO causes older blocks of data to deteriorate, resulting in poor read speeds when accessing older data. Fortunately, Samsung acknowledged this bug and released a fix.
The new firmware's version number is EXT0CB6Q, and it can be downloaded from Samsung immediately. What this firmware does is rewrite older data blocks, ensuring that the data remains fresh, avoiding excessive read-retry processes.
The update can be installed through Samsung's Magician software, or you can run it off of a bootable disk. Either way, be sure to back up your data in case something goes wrong.