Not surprising. The 3GS does a horrible job with Spotlight, you basically have to keep turning it off to get your RAM performance back to iOS 4 'Normal'. I don't mind upgrading but I don't like how people are ' forced ' to get a new iPhone is they want to continue upgrading the OS.
I own an iMac from 2008 and there's absolutely nothing that would make me want to get a new computer after less than 3 years. I mean, what does that say about the iPhone hardware ? That it usually lasts about 2 years before it's no good ?
AT&T is still selling iPhone 3GS and Apple should continue to offer some kind of updates instead of cutting it off completely. They're smart people, they can do it but may they ....