Scores of people with Samsung 840 EVO SSDs have reported slow read speeds with older files. The problem seems to be most pronounced with data that's several months old, and it may be related to voltage drift within the drive's TLC NAND cells. Three-bit NAND is especially sensitive to voltage changes, and if those changes become too large over time, the error correction required to compensate could slow read performance considerably.
Whatever the cause, Samsung has a solution. The following statement just hit our inbox. (Emphasis mine.)
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.
Interestingly, the statement references the 840 EVO specifically. There's no mention of the 840 Series, which is based on an earlier generation of Samsung's TLC NAND. Perhaps it wasn't affected by the same problem.