I hear that if we keep at least one persistent disk of an image around in a zone, future creations are O(1) instead of O(n GBs of copying into the zone). That would speed up our instance creations, especially for Windows.
I've done this by hand so far. (but not for plan9 or linux-buildlet-arm yet) We might want to automate it in the future, though. I don't know if there's a GCE call to inquire about metadata of an image so we know whether we need to create a disk out of it in a zone, though.