An idea regarding back-pressure for iterable sources.

example.js

var Rx = require('rx'),
    Ix = require('ix'),
    log = console.log.bind(console);

Rx.Observable.iterate(
    Ix.Enumerable.range(0, 100),
    10,
    function (x) {
      return Rx.Observable.empty().delay(Math.random() * 1000);
    })
  .subscribe(log);

Observable.iterate.js

Rx.Observable.iterate = function (enumerable, maxConcurrent, workSelector) {
  return Rx.Observable.create(function (o) {

    var i = 0,
        items = [],
        enumerator = enumerable.getEnumerator(),
        done = new Rx.Subject(),
        more = function () {
          if (enumerator.moveNext()) {
            return true;
          } else {
            done.onNext(true);
            done.onCompleted();
            return false;
          }
        },
        next = Rx.Observable
          .if(
            function () { return more(); },
            Rx.Observable
              .defer(function () {
                return Rx.Observable.return(enumerator.getCurrent());
              })
          );

    maxConcurrent || (maxConcurrent = 1);

    while (i++ < maxConcurrent && more()) {
      items.push(enumerator.getCurrent());
    }

    return Rx.Observable
      .fromArray(items)
      .expand(function (item) {
        return workSelector(item)
          .ignoreElements()
          .concat(next);
      })
      .takeUntil(done)
      .subscribe(o);
  });
};