The CPU performance will be the same, but the CPU may be doing more stuff due to more features, making the overall experience slower. Just compare an iPhone 4S running iOS 7 vs iOS 9. The one running iOS 9 will be much slower, there are plenty of tests online and YouTube videos to confirm this. Was it deliberate to make people upgrade? Maybe not. Did it force many people to upgrade? Probably. Was it good for Apple? Obviously. Do they have a reason to stop doing this? Why would they?
If you don't believe me just make a video of your current iPhone doing daily tasks, like typing on the keyboard and scrolling. Then come back 3 years later with the same phone updated to iOS 13 or whichever one will be current by then. Compare it to the video and you'll see that it's gotten slower. Which makes perfect sense, you haven't upgraded your hardware but you've upgraded your software. It couldn't be any other way.