|>>|| No. 22372
>It's worth remembering that mobile is the only place where we're really genuinely getting more power.
This. The iPhone 6s is 20 times faster than the iPhone 4. Apple have made vast performance gains by designing their own silicon from scratch, throwing billions of dollars at the problem. The 6s is faster than most laptops.
Apple aren't actively conspiring to gimp the performance of old devices, but keeping the OS lean isn't a high priority either. Adding features to software invariably degrades performance. It's inescapable - bigger binaries take longer to load from memory, bigger config files take longer to parse. With that said, Apple are clearly working on performance, if only because saving CPU cycles improves battery life. iOS 9.2 has significant performance improvements in many areas.
Devices also unavoidably get slower with age. This is partly because users tend to install more software (meaning more background tasks) but also because of flash ageing. Flash memory performance plays a large role in responsiveness, but flash does wear out with repeated writes. As flash cells start to fail, the memory controller has to work harder to correct errors and reallocate data, reducing performance. It's roughly analogous to a magnetic hard drive becoming fragmented, only permanent.
Discosure of interests: I'm predominantly an Android user, but I do own an iPad.