Have you looked into webassembly and how browsers give access to filesystems[1], there could be hope for a future of high performant filesystem access.
[1] https://developer.mozilla.org/en-US/docs/Web/API/FileSystem
[1] https://developer.mozilla.org/en-US/docs/Web/API/FileSystem
This interface will not grant you access to the users filesystem. Instead you will have a "virtual drive" within the browser sandbox. If you want to gain access to the users filesystem you need to invoke the user by eg. installing a Chrome extension. The relevant Chrome API can be found here.