With the work you and others are doing, a sub-1-second boot seems to be a fast-approaching reality. Hopefully with some template-side-tweaks (or even a native specialized template like debian-15-quick) this should be attainable within a few years.
Maybe I should start a thread asking people for ideas about what could be done with this–i.e. how to leverage the almost-negligible cost of booting (and shutting down) a VM to make Qubes more secure and more usable.