Those are called performance bugs. That was a long time and a lot of software development cycle changes ago.
If it helps I believe the EU just made it illegal two days ago to release updates that make battery life worse. But I could've confused some of the details there.
It was not a bug. I have similar story with my iPhone 4S became unusable with iOS 7. It was never fixed. One day I managed to jailbreak it, rolling back to iOS 6 and was astonished how fast and smooth it is.
They just implemented iOS 7 with much more compute requirements, so only newer phones were able to run it smoothly. The progress of mobile CPUs these old days was crazy, so I don't really complain about it, it was inevitable.
If it helps I believe the EU just made it illegal two days ago to release updates that make battery life worse. But I could've confused some of the details there.