I have built a chrome app[1] that uses the webkitdirectory feature of chrome. No performance issues whatsoever. Also, we have never received a single complaint from our 500k users about its performance.
It would make life easier for developers if Mozilla took an iterative approach. Implement it the Chrome way, then work with Google and Microsoft to implement a new standard.
[1] https://chrome.google.com/webstore/detail/readium/fepbnnnkka...