Images MUST be raw only because of this bug in Cinder RBD driver:
https://bugs.launchpad.net/fuel/+bug/1246219

I'm working on a fix that would make the RBD driver check the image format and revert to full copy and conversion to raw for non-raw images.

For a raw image, @mihgen is right: it is faster with RBD backend because it is cloned instead of being copied, which is just a metadata change.