Nowadays, as electronic gadgets are more entangled in our lives than they have ever been, when a new version of our favorite gadget comes out, there are several things that start to happen.
Suddenly the phone/tablet we used to like so much seems inadequate, slow or just plain old. The good news is: we're not alone. According to Google Trends data cited in a recent New York Times article, search requests for the keyword combination "iPhone slow" have peaked six times in the past seven years, each of those peaks occuring right after Apple released a new iPhone model.
There are two possible explanations for these spikes: the first one being the (psychological) phenomenon described above. Once people know a newer model of their phone is out, they suddenly feel like their old one is getting slow. The second explanation, and maybe the more plausible one, is that iPhone releases often coincide with the release of a new version of iOS, the iPhone's operating system. While Apple has always made a point of making the most recent version of iOS backward compatible with older iPhone models, the new versions tend not to run as smoothly on the hardware of older models. So it seems plausible that people would update to the new iOS version, encounter performance issues and turn to Google to find out what's wrong with their phones.
Apple is in the comfortable position to have full control over both the hardware and software of their devices. It's a position that forces Apple to master a balancing act though: On the one hand, they need to cram more features into iOS because better hardware alone won't sell millions of iPhones. On the other hand, they need to ensure backward-compatibility without sacrificing too much performance, because if they don't, people will accuse them of planned obsolescence.
This chart illustrates how Google searches for the keyword combination “iPhone slow” tend to peak right after Apple announces a new model of its popular smartphone.
You will find more statistics at Statista