Are there performance issues with iOS 6 running on iPhone 3GS?
According to this ArsTechnica article


  iOS 6 doesn't make the iPhone 3GS any slower or more difficult to use than it was before, which should be good news to anyone who keeps theirs around as a secondary or backup phone—indeed, it's pleasantly surprising how many of the refinements and improvements make their way down to Apple's oldest-supported piece of iOS hardware. However, that hardware has been surpassed so thoroughly by other iOS and Android handsets at this point that it's difficult to recommend it as a primary handset. You don't have much to lose if you upgrade a 3GS to iOS 6, but there's a lot more to gain by investing in a newer device.