These seems to be the norm Apple likes dropping off it's old devices (especially ones that were very popular) on a version that is unusably slow and seems to be a motive to get people to buy new devices. I'm not sure why the iPhone 4S is so slow on iOS 8, I feel like Apple is doing it on purpose iOS 8 is really not that different from iOS 7, let's go through Apples devices that they discontinued and see how they run shall we?
iPhone, 3.1.3: Decent
iPhone 3G, 4.2.1: Unusable
iPhone 3GS, 6.1.6: Decent
iPhone 4, 7.1.2: Unusable
iPhone 4S, 8.X: Unusable
iPod Touch 1G, 3.1.3: Decent
iPod Touch 2G, 4.2.1: Unusable
iPod Touch 3G, 5.1.1: Fine
iPod Touch 4G, 6.1.6: Usable
iPod Touch 5G, 8.X: Unusable
iPad, 5.1.1: Fine
iPad 2, 8.X: Unusable
iPad Mini, 8.X: Unusable
iPhone, 3.1.3: Decent
iPhone 3G, 4.2.1: Unusable
iPhone 3GS, 6.1.6: Decent
iPhone 4, 7.1.2: Unusable
iPhone 4S, 8.X: Unusable
iPod Touch 1G, 3.1.3: Decent
iPod Touch 2G, 4.2.1: Unusable
iPod Touch 3G, 5.1.1: Fine
iPod Touch 4G, 6.1.6: Usable
iPod Touch 5G, 8.X: Unusable
iPad, 5.1.1: Fine
iPad 2, 8.X: Unusable
iPad Mini, 8.X: Unusable