Slower read speeds on old data is indeed a symptom of the SSD controller having to use its slower and more thorough error-correction mechanisms to recover data.

Slower read speeds on old data is indeed a symptom of the SSD controller having to use its slower and more thorough error-correction mechanisms to recover data. However, since the first few generations of Microsoft Surface machines, the SSD market has made several relevant changes: - a switch from storing charge in a conductive floating gate structure to storing charge in a non-conductive charge trap layer (Intel's flash is the one holdout here) - a switch from planar to 3D fabrication, allowing a huge one-time increase in memory cell volume and thus the number of electrons used to represent each bit, and also opening up avenues of scaling capacity that don't require reducing cell volume - dedicating far more transistors to error correction in SSD controllers, greatly reducing the performance impact of correctable bit errors but also enabling the use of more robust error correction codes