I can second this. My wife has a personal 6S, her work phone (they pay for the plan, but not the phone) is a 5S we had laying around. She hates going on international business trips with that phone (we could swap the SIM), but "it's so slow", "it doesn't work", "i hate this phone" are common phrases with extended usage.

I like how every generation of every phone is marketed as "This time, the camera does true colour!" - I wonder how much real difference there is between gens.

Downgrading is always hard. Even minor differences in speed/fluidity will seem much more accentuated.