Samsung recently issued a fix for read speed issues with old data on its 840 EVO SSD. The company first claimed this issue was unique to the 840 EVO but now Samsung acknowledges that there are also issues with the 840 Series SSDs. The issue seems to confirm that the TLC NAND memory is the cause of the problem, the higher bit density makes TLC memory more sensitive to voltage drift which makes data harder to read over time. This results in more work for the error-correcting mechanisms which severely hurts read performance of old data.
We asked Samsung how it determined the 840 Series was immune, and we're still waiting on a response. However, German site ComputerBase has published a statement from the company that seems to acknowledge a problem with the older drive. The Google translation is a little rough, but PC Perspective has a better one that says "accessing specific data with units of SSD 840 could lead to lower reading performance." Samsung is reportedly working on a solution, but there's no associated timeline.
Interestingly, the translated Samsung statement goes on to say, "Due to different technologies the PRO-series (840 PRO and 850 PRO) are not affected." That technology difference is the number of bits per cell: MLC drives like Pros have two bits, while the 840 Series and 840 EVO use three-bit TLC NAND.