I don't know much about the older versions (last time I have played with iOS was iOS 7 I think), but I have recently got an iPhone 5S for development with iOS 1

I don't know much about the older versions (last time I have played with iOS was iOS 7 I think), but I have recently got an iPhone 5S for development with iOS 11.2 and it's definitely one of the most glitchy mobile OSes I have ever used - and I've used plenty of community managed AOSP "ROMs", Replicant with missing drivers and some Openmoko distributions. I didn't expect much, but I honestly didn't expect it to be so bad. And I'm not even using it as a phone.