As codebases get larger, inefficiencies are bound to happen. Having explicit imports that I can go look up and see where they are is better for resolving this, because you can trace how every module gets imported and time them. Having interpreter code that runs with lazy loading that is all hidden is not the way to solve this.

As for external libraries, you import them in places where you need to use them only to avoid the same pitfalls. Its also pretty easy to analyze the import process within those libraries, and then again import specific submodules only that limit what actually gets loaded.