So you got an old iPhone and did a software update and you are thinking why my iPhone is so slow. Well, Apple makes software updates for the new models which means that they are designed to work with the new hardware combined. So that would be one of the reasons that your iPhone is slow. In my shop i hear conspiracy theories very often and one of them is that apple makes software updates to get those people buy the new iPhone because their old is slow. This are my theory opinion on this topic, you tell me whats your theory ?