>Even if iOS 2 were designed more for the 3G than the original iPhone, it couldn't have required significantly more resources because no more became available!
That wouldn't matter if Apple were simply adding code that tested the hardware and manually added slowdowns on the older model. Your observation could be seen as lending credence to the conspiracy theory.
The spikes for iOS 4 and iOS 7, which are relatively larger than for the other releases, may be justified. iOS 4 brought limited multitasking and more significant OS-level changes than iOS 3, and iOS 7 brought the visual refresh which is more CPU- and GPU-intensive.
[1] Or due to the upgrade process itself, rather than changes in newer OSes, slowing down phones