AnandTech writes Samsung's SSD 840 and 840 EVO suffer from a weird read performance bug that affects old data. A graphical illustration of the problem can be seen in the HD Tach screenshot below this article, the bug causes the read performance of old blocks of data to slow down to a crawl. The bug only manifests itself with old data (like >1 month), once the data is overwritten the read performance of these blocks returns to normal.
Samsung says they're aware of the issue and promise the bug will be fixed via a firmware update:
I just got off the phone with Samsung and the good news is that they are aware of the problem and have presumably found the source of it. The engineers are now working on an updated firmware to fix the bug and as soon as the fix has been validated, the new firmware will be distributed to end-users. Unfortunately there is no ETA for the fix, but obviously it is in Samsung's best interest to provide it as soon as possible.
As for theories about the cause, The Tech Report offers two possible explanations. The first theory is that there may be a bug in the background garbage-collection and wear-leveling algorithms that shuffle data around the disk.
The other theory is that it may involve a change in cell voltages over time. Flash drives write memory by tripping electrons inside a floating gate, electrons can leak out of the cell over time, though, and the cells are also prone to interference from nearby cells. The TLC NAND memory used in the affected disks is more sensitive to voltage changes than MLC memory because it must differentiate between more possible values within each cell. If this is the case, the slow performance on old data could be caused by the extra work required by error correction mechanisms to read out the old data.