Script streamers can easily become stalled waiting for data which has come in from the network, but hasn't had a chance to bounce off the main thread yet (e.g. because other JavaScript is executing or HTML is being parsed). The main thread bounce is currently necessary because the data goes via the ScriptResource.

Instead, script streamers should receive their data directly from the network (IO?) thread, concurrently with that data being sent to the main thread (for processing by the ScriptResource). Potentially this would mean moving the whole ScriptResource off-thread, with a main thread dispatch only when the resource is completely loaded.