It's not just Apple - it's the developers who release updates to their apps which, because they have the latest gear, are optimised for the latest hardware. I have an iPad2 and only recently upgraded it to iOS9 after 6? (I think, the one where they introduced the wobbly effect) - it got slow before I upgraded it because I updated all the software on it incrementally. It started lagging before I upgraded, indeed it was the reason why I upgraded the operating system. It didn't make any difference.