[richardwhiuk]

Apple occasionally release upgrades which the hardware really isn't capable of supporting. I think the worst example was the iPhone 4, which was given the iOS 7 upgrade. iOS 7 was basically only suitable for multi-core capable devices, and the iPhone 4 was the only device it was released on (the same generation iPod and iPad didn't get the update).

[MBCook]

Apple tries really damn hard to make their stuff run well on all devices and it's very impressive how well they support them.

But if your device is a few years old… You're right that you may want to wait and see what the reviews say. Sometimes the updates actually make the phone feel faster, but sometimes (like iOS 7 with all the new transparency effects) it definitely can be a worse experience.