Instead of the serial end() - every item in the source list is processed after the previous one is ready - endParallel() makes parallel calls to a generator that uses filters that can be executed separately.

Parallel driver for async.js

var async = require('asyncjs')


async.plugin({

    endParallel: function(callback) {
        var source = this.source
        var e
        var min_i_err = Infinity
        var first_e = null
        var values = []
        


        var counter = 0
        
        var handler = function(i) {

            return function(err, value) {

                e = err
                if (err) {
                    if (i < min_i_err)
                        min_i_err = i
                        first_e = err
                
                    /*if (err == STOP)
                        
                        callback && callback(null, last)
                    else
                        callback && callback(err, value)*/
                }
                else  {
                    values[i] = value
                }

                process.nextTick(function() {
                    if (--counter == 0)
                        callback && callback(first_e == async.STOP ? null : first_e, values[values.length - 1])
                })
            }
        }

        do {

            source.next(handler(counter++))
        } while (!e)
    }
})


var t


async.list([
    function(next) {

        t = +new Date
        async
            .list([1,2,3,4,5,6,7])
            .filter(function(i, next) {

                time = Math.round(Math.random() * 1000)
                console.log(i, 'yet '+time)
                setTimeout(function() {

                    console.log(i, 'now');
                    next(null, i < 5);
                }, time)
            })
            .end(function(e, v) {

                console.log('serial', arguments[1], +new Date - t)
                next()
            })
    },
    function(next) {

        t = +new Date
        async
            .list([1,2,3,4,5,6,7])
            .filter(function(i, next) {

                time = Math.round(Math.random() * 1000)
                console.log(i, 'yet '+time)
                setTimeout(function() {

                    console.log(i, 'now');
                    next(null, i < 5);
                }, time)
            })
            .endParallel(function(e, v) {

                console.log('parallel', arguments[1], +new Date - t)
                next()
            })
    }
]).call().end()

$ node async_parallel.js 
1 'yet 504'
1 'now'
2 'yet 840'
2 'now'
3 'yet 160'
3 'now'
4 'yet 470'
4 'now'
5 'yet 130'
5 'now'
6 'yet 928'
6 'now'
7 'yet 126'
7 'now'
serial 4 3188
1 'yet 85'
2 'yet 582'
3 'yet 828'
4 'yet 922'
5 'yet 315'
6 'yet 974'
7 'yet 486'
1 'now'
5 'now'
7 'now'
2 'now'
3 'now'
4 'now'
6 'now'
parallel 4 980