The Tech Report received word that Samsung is working on a firmware fix to resolve issues with very slow read speeds for old data on the 840 EVO SSD. Unfortunately, Samsung didn't reveal the cause of the problem but it seems it's specific to the 840 EVO and not the 840, which uses an earlier generation of TLC NAND flash memory. One of the theories making the rounds is that the performance issue may be related to voltage drift within the 840 EVO's TLC NAND cells. Three-bit-per-cell NAND flash memory is more sensitive to voltage changes, so it's speculated that if those changes become too large over time, you may get a considerable performance slowdown due to the extra work required by the disk's error correction algorithm.
Here's Samsung's statement:
We acknowledge the recent issue associated with the Samsung 840 EVO SSDs and are qualifying a firmware update to address the issue. While this issue only affects a small subset of all 840 EVO users, we regret any inconvenience experienced by our customers. A firmware update that resolves the issue will be available on the Samsung SSD website soon. We appreciate our customer’s support and patience as we work diligently to resolve this issue.