It's likely still a memory issue though rather than anything to do with the CPU. As with the 4th gen touch, it's smaller display on the 3G likely means it has enough memory for enough of iOS 6 features compared to the iPad 1.
I think you're probably right - to give some context, the iPhone 3GS and iPad 1 have the same amount of RAM (256MB), but the iPad has to push roughly four times the amount of pixels as the 3GS.
