SSDs usually calibrate changes in the statuses of cells over time via the flash management software algorithm. Due to the error in the software algorithm, the 840 EVO performed read-retry processes aggressively, resulting in a drop in overall read performance. This only occurs if the data was kept in its initial cell without changing, and there are no symptoms of reduced read performance if the data was subsequently migrated from those cells or overwritten.
Samsung has now released firmware EXT0CB6Q that fixes the problem. There is also a utility that you can run that will restore the drive's performance. Both can be downloaded here:
http://www.samsung.com/global/business/ ... loads.html
I'm currently in the process of updating the drive in my laptop.