> Some of the difference is due to Safari being a bad browser (probably intentional), but a bigger part is that only having one thread makes it really difficult to have a responsive UI

Interestingly Safari is actually generally better than other browsers for running a responsive smooth UI. Not sure how much of that is the Safari engine and how much of it is better CPU on iPhones. But even on a first generation iPad or an iPhone 4 it was possible to get 60fps rendering fairly easily. The same could not be said for even higher end android phones of the time.