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).
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.
