Although I don't have an iPhone, I do remember my work colleague complaining about her iPhone 4 running unusually slow around the time of the iPhone 5 release. On the other hand, I never thought of asking whether she installed a new iOS version, but interestingly she hadn't complained about its performance since then. This is different to her previous phone (Galaxy Ace) that once it slowed down, she was complaining about its performance right up until she got the iPhone.
Now that this article is out there, I'm sure there is going to be someone who has an iPhone collection interested in checking this out, e.g. run daily benchmarks (page load tests, etc.) and watch out for any fluctuations when the iPhone 6 launches. If they have a few iPhones, they could even upgrade some to the newer iOS (once launched) and see what impact that has also.
The source New York Times article is worth looking at if you can get past the 'Subscribe to view' screen as it shows some detailed graphs that clearly show the 'iPhone slow' search spikes against the iPhone release dates. I copied & pasted the link into a new Chrome window and the full article appeared.