I think this is a bit different. Even if the file reading code was reading the maximum amount of available data it would still have to store most of it in a loc

I think this is a bit different. Even if the file reading code was reading the maximum amount of available data it would still have to store most of it in a local buffer and then yield only one line in a promise which would mean only one line was being processed per event loop tick. There's one more thing you should be aware of here that can catch a lot of developers out. On most POSIX systems, file reads from disk do not return EAGAIN but will in fact block if the disk is too slow returning the data back. EAGAIN is only meaningful on things like pipes, sockets, etc. The only way I'm aware of to do disk access truely asynchronously is using the POSIX AIO interface.