All computing devices are prone to slowing down over time, as their memory fills up and extra software is installed, but the way iPhones use their memory makes them less prone to this than most.
It's possible that it just feels slower than it used to, because you've got used to what initially seemed dazzlingly quick operation, or have started to compare it to friends' newer, faster models.
But sometimes it's possible to make a direct comparison: when there's an app you used to enjoy but has since become unusably juddery. Such a situation is what led us to write this feature. The wonderful, super-fast game Super Hexagon demands lightning reflexes to survive, but the stuttering graphics on our iPhone 4 now make it impossible to play. We've also been seeing problems with the iPhone 4 handling Temple Run 2.
Here are some great methods to speed up an iPhone 3GS, iPhone 4, iPhone 4S or even iPhone 5, iPhone 5c or iPhone 5s, if you're not happy with the way it's running.