The lxc publish command’s job is to take an existing instance and convert it into a compressed image file. This necessarily means that to use that image it needs to be decompressed to be used.
However, depending on the storage pool type you are using, the image will only be unpacked once, for the first instance created, and subsequent instances created using that image will re-use the unpacked image volume.
See Images not syncing to a restored cluster member, image copy fails with Error: Failed remote image download: - #10 by tomp for more info.