It works in two steps: the first step resolve to "unqualified paths" (which are just the paths within the cache without the index.js/extensions resolution). This step is entirely static, no filesystem involved here.

The second step converts the "unqualified paths" into "qualified paths", and is basically the index.js/extensions resolution. We currently access the filesystem in order to resolve them (just like Node), because we didn't want to store the whole list of files within our static tables (fearing that it would become too large and would slow down the startup time because of the parsing cost).

So to answer your question: we get rid of the node_modules folder-by-folder traversal, but decided that the extension check was an acceptable tradeoff. We might improve that a bit by storing the "main" entries within the tables, though, which would be a nice fast path for most package requires.