For now, the latter:

> Other formats, such as JSON files and DuckDB's native database format, use synchronous I/O.

For the future:

> Most of its benefit comes from accessing remote data, but cold local reads can also benefit, albeit less. Next, we plan to add async reads for JSON and DuckDB-native files, as these are the two other formats most relevant to DuckDB core. Formats that live in out-of-tree extensions are not on the roadmap yet.