As data ages on the Samsung 840 EVO, the SSD's retrieval speed of those old files can drop dramatically, sometimes even to speeds below 50 MB/s on files that are a few months old.
This problem went undetected for about a year because regular SSD benchmark programs write their own test file (to test write speeds), and then immediately turn around and read that same file to test read speeds. This means that all the traditional SSD benchmarking programs never see this phenomenon because they are always reading a fresh file. Since testing for this strange SSD behavior would require a new method of benchmarking, I wrote a new benchmarking program that would specifically test for an age/speed correlation by reading files already on the SSD, and then display its findings graphically.
The first time you run SSD Read Speed Tester, it may ask for administrative privileges to install a small runtime component (comctl32.ocx). The program itself is standalone/portable. If the component is already installed (or you manually install it yourself), the program won't ask for administrative privileges.
SSD Read Speed Tester saves a PNG picture for upload on this thread (if you're so inclined to share your results with us), and two TSV (tab separated values) files for further analysis for those who want to have the raw data for importing into spreadsheet/graphing software. Please be advised that one of the TSV files contains the pathname of every file tested. I am only interested in the picture.