Comment 8 for bug 1796137

repeat boots could I guess be slow as a result of bug 1786165.
The snaps that are seeded are never removed. perhaps snapd doesn't realize (or has to re-check) to verify that they are not new.

I'd suggest debugging and re-create in lxd itself.
Doing so allows you to jump into the container before networking is available and push/pull files easily.

$ lxc launch ubuntu-daily:cosmic my-test
$ lxc exec my-test /bin/bash
...