Are you sure you're up to date on the latest additions to this API? This wasn't in the MVP:

> The origin private file system provides optional access to a special kind of file that is highly optimized for performance, for example, by offering in-place and exclusive write access to a file's content.

https://web.dev/file-system-access/#accessing-files-optimize...

It was originally going to be a separate high-perf "Storage Foundation" API, but that was merged with the File System Access API.