I have 45 apps on my iPhone 5c and now when I press the home button it sometimes takes 4 to 5 seconds to open up all my apps. Also I had a couple of crashes on the 5C but that's not as bad as on the iPad air which is about once or twice daily. I think iOS 10 has some more polishing to go through before I would consider it as reliable as iOS 9 was towards the end. But overall it's more than usable.
I did have a situation where my iPhone answered a call and I did not press any buttons. It rang once I saw the name on the screen and It just decided to answer. By the way under the phone settings, I love the new Call Announce feature.
That said, my buddy also has an iPhone 5c and he has absolutely no problems, pages load fast no freezing or jittery pages etc.