Its still fast for many reasons:
IOS 5 on 4s is just like iOS 9 on 6S (Extremely fast)
iOS 6 on 4s is just like iOS 10 on 6S (Very Fast)
iOS 7 on 4S is just like iOS 11 on 6S (Fast sometimes, But slow in some areas, slightly battery drain}
iOS 8 on 4S is just like iOS 12 on 6S (very Slow, App crashing sometimes, Moderate battery drain
iOS 9 on 4S is just like iOS 13 on 6S ( Extremely slow, App crashing on heavy apps, Huge battery drain)
My money is about iOS 13 might be the first iOS to have 2 Gb ram Only with A9 Processor. meaning no A8/A8X is supported even the iPad Air 2 or mini 4 cuz if they did the iPad 3 (a5X) just like iPad 4 and iPhone 5/5S with doubled the ram of A5 should have gotten the iOS 10. But unfortunately it isn't.