The processing power (CPU) of the 4th generation iPad is rated at twice the speed of the 3rd generation, and the 5th generation iPads are even faster still! We've observed that the latest generation iPad "feels" quite a bit more snappy. There are no hardware-specific optimizations made on our end to take advantage of the new hardware.
That said, the most significant speed gain is realized by folks upgrading from the first generation original iPad. RAM and processor speed are very constrained in the iPad1, and we've had to make some optimizations specific to that hardware so that Photosmith can work. We strongly urge iPad1 users upgrade to something newer, even if it's just iPad2.
Camera raw image processing is much faster when used with the latest version of iPad.