Techcrunch did a fun test where they placed all versions of the iPhone next to each other and ran a speed test. You can see the video here, the the main takeaway is:
After the 4S, the speed differences for basic day-to-day activities (like loading a page) start to get pretty slim with each next generation. Once you’re on the 5 and later, the difference is hardly noticeable. The differences are obviously going to be more visible for the more intensive things (like 3D gaming), but for general day-to-day stuff like browsing, there’s not really room to make huge strides.
The 4s was released on 4th October 2011 so it is just about two years since iPhones got any faster. Screens and cameras have gotten a bit better since then but if speeds aren’t improving then the competition becomes much more about software (hence the big fuss about iOS 7) and about price. With the exception of the Nexus 4 (which was most likely heavily subsidised by Google) high end Android and iOS smartphones have been consistently priced at £400+. That may now start to fall.