The reason I am not convince CPU is the main factor of lagging in newer IOS because I have both a ipad 4 and ipad air. Both running on 1G ram. As far as performance goes, they run equally bad on iOS 9, although ipad air have a much faster CPU.
To illustrate CPU is not the factor, here is the video on iphone 4S (iOS 9) vs iphone 4 (iOS 7). Both having 512Mb of ram, but 4S is significantly faster on the CPU.
To make the long story short, given both having the same amount of ram, the iphone 4 running iOS 7 with a slower CPU runs smoother and faster than the iphone 4S on iOS 9 with a faster CPU.