Now I am curious if, in the real MDN production site, serach-index.json loading is triggered by the execution of /static/js/autocomplete.js, when their download should really be started in parallel by the shim.

Many websites leave a lot of performance on the table because of such behaviors.

My hypothesis is that, since this is easier for the developer, and works good enough, not many people really care. But these things add up, and the web becomes slower and slower.