^^
So, again, as I was saying. Safari is limited in how much RAM it can use, even moreso when other applications need the memory.
It's a memory limitation. Try running Snow Leopard and Safari with only 512MB of memory and see how well it treats you.
Oh ok, yes that's correct. It's both issues, really. Apple has done well to allocate RAM usage across the devices, but there was never, EVER enough preinstalled to accommodate 4.0. Not on the iPad certainly and barely enough on the iPhone 4.