Samsung has released a firmware update for its 840 EVO SSD, that corrects slow read speeds with older data. "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 said.

Using Samsung SSD Magician software you can flash the SSD's firmware with the new EXT0CB6Q revision. Then, it reboots the system and completes the restoration process - not data will be erased.

Samsung recommended running HD Tach to evaluate the fix.