It would be interesting if instead you added a syntax whereby a module could declare that it supported lazy importing. Maybe even after running some code with side effects that couldn't be done lazily. For one thing, this would have a much broader performance impact, since it would benefit all users of the library, not just those who explicitly tagged their imports as lazy. For another, it would minimize breakage, since a module author knows best whether, and which parts of, their module can be lazily loaded.

On the other hand, it would create confusion for users of a library when the performance hit of importing a library was delayed to the site of usage. They might not expect, for example, a lag to occur there. I don't think it would cause outright breakage, but people might not like the way it behaved.