I can't say for sure unfortunately as my only previous experience of Ubuntu on a Chromebook was via Crouton on a Samsung which, being ARM, was somewhat limited in what you could do with it.

Elementary on the other hand I run natively via dual boot. It's nice and slick, and I much prefer Pantheon to Unity, but I can't really give you a direct comparison on this device.

I don't have a Chromebook, but I do have an HP netbook running an AMD APU. Elementary OS does run better than Ubuntu on it, but granted, it has a decent video card. I've no idea why it was so much smoother in regular usage, but it is!