Of course, iPhones slow down over time. All electronic devices involving computers will slow down as their operating systems acquire new features designed to work better on newer models. This is simply a natural evolution in the development of both the operating system and the product line, and it's not a cause for concern. Chances are that one of the main causes of your iPhone becoming slower is iOS location services.
After all, there are some apps that need to know your location, how is Google Maps going to give you directions if it has no idea where you are? But there are many more apps that don't need to know your location or that can be changed to “while using the app”.