If you bought it in march you still have seven months of warranty, no need to rush here; and before bringing it back to Apple (unless its like two blocks away, in which case I urge you to drop the forums and go straight there), I'd ask just how much you know about how to use it. I mean, I don't want to sound like you don't know what you're doing, but it slowing down can easily be imparted to management. You should perhaps tell us more about what you find to be slow. It could be something simple like too many apps active in the background - although the 4S is hard to slow down in that aspect. It could be that you topped up your storage to its very limit - my friend saw his iP4 slow down when he got to the point he only had a couple of MB's left - but then again I guess the 4S would fare better in that situation, perhaps with no effect at all on speed. Like it was mentionned, if you're jailbroken, it can be many things : bad or outdated package, adding a bad repo, having tweaks or themes that interfers with each other, having too many of them... And perhaps - but I may be way over my head here - it is something in your native settings at large that get the phone to be always searching and working on too many things at the same time. I don't know about that, but I guess if you downloaded hundred of apps and everything is in your notification center (for example) it can really take a toll on speed. And of course it could be a combination of all of that - and else.
Whatever it is, and whatever you need to do, I wish you good luck, and feel free to post more information. And Sean is right, if it's a problem with the device, no need to worry, Apple will see to that.