Depends on the import tree. The way I understand it, this:

    import { x } from '/a.js';
    import { y } from '/b.js';

Does not take longer than this:

    import { x } from '/a.js';

Because the message to the server "Give me b.js" goes out in the same network packet as "Give me a.js" and the data of b.js comes back in the same packet(s) as the data of a.js.