That's an easy one
iOS 8:
QuickType - the contextual predictive typing feature which predicted your next word. I think the A5 wasn't quite up to the task and turning this off removed most of the keyboard lag on the 4s
Continuity - the handoff feature that was always running behind the scenes (another feature using CPU cycles)
iOS 9:
Proactivity - The operating system is more contextually aware of information (such as time and location), and can provide the user with information ahead of time. For searching, the proactive intelligence can display instant results in a widget-like format, including weather, sports, news, and more. (Yet another feature using CPU cycles)
Sure iOS 7 and iOS 9 may look the same on the surface, but it's the under the hood changes that really made the difference.