If you're the owner of one of Samsung's 840 EVO solid-state drives, chances are you're familiar with the serious speed degradation problem that can arise over time, in some cases throttling it to 1/10th of the speed it should be. Well, the good news is that a fix is available; the bad news is that it's not even clear at this point if it actually squashes its bug for good.
To update, you'll need to grab the latest version of Samsung's Magician, versioned 4.6. There is a caveat, though: it seems that Samsung is preferring to have people update on as small a scale as possible at first, as it's limiting the amount of downloads that can be snatched each day. If you're impatient (and I'd be right there with you), PC Perspective has a list of mirrors that could remedy that problem quick.
Unfortunately, Linux and OS X users are currently out-of-luck, as Samsung hasn't released the needed updater tool for those OSes. However, if you happen to dual-boot, and have the SSD acting as a secondary drive, you'll be able to update it that way.
If you can muster a bit more patience, I'd recommend holding off for a couple of days to see how this all plays out. This bug is more persistent than most, so it's hard to say that this actually definitively fixes the problem. If you do update, be sure to backup your data - never take chances.