Here are some results from my benchmark [1]

results for 20000 parallel executions, 1 ms per I/O op

  file                   time(ms)  memory(MB)
  promiseishBluebird.js      1076       76.08
  async.js                   2634      113.79
  promiseishKew.js           2654      123.50
  promiseishQ.js            56508      804.21

note: async.js is basically waterfall from https://github.com/caolan/async

[1]: https://github.com/spion/async-compare