One of the problem with the way we built WebPageTest today is that we don't use the built in ability to scale. WebPageTest can be configured to start a new test agent if the queue is longer than configurable X It's not needed today but when we add more and more URLs we will hit the limit and it would a nice feature to use.
Today when we add tests to WebPageTest we do it in a series, we add one test, waits for it to finish and then adds the next. We need to change that so we add all at the same time (from a batch) and then wait for them to finish.
Change that is easy but it will take som time to test. We use the caolan/async library today for making the requests in a series, we change that to use parallell instead + set the timeout time for waiting on a test to unlimited. That should do, but we need to test :)