>Synchronous reading means you have to read a given chunk in before you can process it, which means that if you're not reading out of cache that you're going to

>Synchronous reading means you have to read a given chunk in before you can process it, which means that if you're not reading out of cache that you're going to stall on the spinning disk, but synchronous code doesn't require you to read the whole file at once. It's implicit that stalling the node.js event loop is not allowed, so a synchronous "line" generator or iterator would block the event loop when it has exhausted the data read so far. The node.js event loop cannnot suspend the current task and process other tasks while waiting for more data to be read. Edit: The point of the author's post is that async iterators generate a Promise (and thus a yield to the event loop) for every element, even when the element is already available synchronously. Even if the generator has read three lines in the latest call to read(), it must still yield three promises of one line each (each involving one yield back to the event loop) before all three lines are processed. Edit 2: That said, it's not hard to fix this, and the author actually comes close to the solution but doesn't realize it. They say the result of the async iterator in the spec should contain an array of values, but it's just as easy to write the "line" iterator to yield an array of lines as its values without changing the spec.