Just yesterday evening I set up an old 5C for my mother to use. She has had an iPhone 4 for so long now I just couldn't watch her struggle with it anymore. Obviously she was quite happy with the 4, but I think it pained me more than her.
Anyway, I was actually very pleasantly surprised at how well iOS 10 runs on the 5C. Everything seemed to be working well, and that was right post-install and setup, so I imagine it will have been even better today. Obviously comparing the 5C to the newer models there are striking differences regarding performance, but nothing all that jarring.