So I was browsing reddit yesterday and came across this - http://www.overclock.net/t/1507897/samsung-840-evo-read-speed-drops-on-old-written-data-in-the-drive .
Owning a few of them myself as well as using them in friends/relatives builds it's kind of turned me off. If you own one be prepared for performance decreases with old data (specifically) on the ssd.
Still rocking my Crucial M4 in my main system which also had it's issues (firmware lulz). So I got that going for me, which is nice.
I have heard this before, in some cases it was some people not being plugged, not into SATA 6, but into SATA 3 I may have misread, I was only skimming it after all.
I seem to recall this came up in some intensive SSD testing about a year or so ago. It turned out to be some kind of caching issue in that case. I will give it a read though at some point, as I run Samsungs and have just bought some more. I can't say I've ever had this issue with any of mine, and skimming through it seems like people managed to solve their slow downs by fixing some caching issues with Windows (just waiting for Linux to take over the world at this point).