Given the level of animation that the 3G can handle in games and so forth, I'd say this is ridiculous and just an attempt to artificially limit what older devices can do. Either that or the code behind iOS 4 is written poorly and actually does require additional CPU cycles...
Honestly, if you've ever done any graphics programming at all, you'd know that making the required adjustments to the home screen would be trivial in most environments. Battery life? Christ, this isn't rendering an hours worth of HD video at 30fps every time you swipe into your phone!