A full-fledged OS, especially one with a lot of drivers and services, will not be as fast and will use more power. But I think you should take a better look at other differentiating aspects of Google OS, namely security, stability and ease of use.
There are videos on the Chromium page about the details, but some highlights:
1) There is only one version of the OS, the interface is very simple and you can't break the OS in any way (ease of use, stability)
2) The OS itself and user data are kept on separate partitions which makes it easier to update the system and limit access (stability, security)
3) There are two copies of the system partition so it's not a problem if the system update fails (stability)
4) The user data partition is encrypted (security)
5) They use custom firmware that, when you boot the machine up checks if the OS partition was modified and -- as far as I understand -- can automatically restore the system to original state (security)
6) All system updates are downloaded, verified and installed automatically without any user intervention (ease of use, security)