"When SSD Performance Goes Awry"
http://www.techspot.com/article/997-samsung-ssd-read-performance-degradation/
According to Samsung, the algorithm that adjusts the voltages used to read the NAND as it ages had a problem which meant that data previously written but never rewritten became harder and harder to read. The speeds to read such a file could plummet from 500MB/sec to below 50MB/sec, a 10x reduction in performance!
Samsung claims the read performance degradation issue only exists in the NAND used by the popular SSD 840 EVO. However there are OEM versions of the drive that use the exact same NAND.