I hope you know about Samsung's EVO performance loss issue?? Non the less the issue has been solved, please upgrade the firmware as soon as you can, otherwise those data that has been just sitting there for more then 4weeks or more will give read speeds around 80mb/s, It happens to every Samsung TLC nand that i know of, I would have never noticed that unless it wouldn't had happened to me, my 120gig EVO was booting windows very slowly like 1 minute or so cauz I didnt even use my evo around 2-3months, Khair point is update the firmware it will be as good as new or rewrite the data which is not been used for more then 3weeks just before 4weeks so that you wont get the performance loss like i had experienced.
Its really nice to see samsung solved this matter with in 2weeks after it was proven to them, you can read more at this on PC perspective,
Samsung 840 EVO Performance Restoration Tool preview - Getting EVOs back up to speed | PC Perspective