Blame Metal if anything - I have noticed Metal really struggles with moving graphics (Emoji keyboard scrolling stutters a lot).
OpenGL never had these issues. For all intents and purposes iOS8 and iOS9 basically looks identical, but performance in really basic areas (scrolling Settings for example) just does not perform as well - the system hasn't been radically overhauled, or some new high definition graphics put in place to cause it. The key change has been the shift from OpenGL to Metal, and we are all suffering as a result (even the newest phones have stutter, so it's not hardware).