recently there was an interesting discussion about this SSD at overclock.net, and other sites. in short due to a very weird bug, older files, usually a few months old, stored in 840 EVO will get very low read speed. fresh files are not affected.
however if you did a search about Samsung 840 EVO getting much slower over time, actually similar incidents have been reported on and off, but capturing much lesser attentions.
one of my laptops, a Dell XPS 9530, has 2x 500GB 840 EVO. i just checked them out and it turned out to be true. the laptop acted very sloppy these days. an app that could be loaded in a second previously needs more than 10 seconds to do the same now.
the (only?) quick fix to this, believe it or not, is to run a disk defragmentation on the ssd.
UPDATE: a new performance restoration tool for 840 EVO was released on 15 October 2014. you only need to run it once. it install a new firmware, reboot, and then recondition your drives.
hopefully this will fix the issue for good.