I don't think it's deliberate, but certainly as Apple issue updates it does tend to slow things down a lot.
I have read loads of comments from people saying the iPhone 5s lags and is slow and it might be time to upgrade.
Seriously? The flagship phone until a few days ago is slow? No it isn't. If it is, something has gone very badly wrong. Nobody connected on it being slow when it came out and it still isn't given the fact that developers all write code to make apps and games run on even older hardware.
Suffice to say people will suddenly find their iPhone 6 appears slow in a year, which is madness.
The question is why Apple isn't called to task for why it doesn't make sure OS updates are fully tested and optimised so as not to impact older models. Here it IS as if Apple isn't so bothered as people will upgrade, such is the loyalty of many users.
The iPhone 5s chipset isn't much different to the 6.