if this._readableState.bufferSize is very small,  buffer of fs.read() gets small so that it takes more time to finish reading as in https://github.com/joyent/node/blob/master/lib/fs.js#L1491

If options.bufferSize is 0, it is set to 16k by default  as in https://github.com/joyent/node/blob/master/lib/_stream_readable.js#L35