We have seen our outbound external queue pile up before and virtually come to a stand still for almost 3 days. When that happened, we had to log a case and the Cloud Operations team needed to "open the flood gates" and push the calls through. Theoretically speaking, once your program/internal event triggers the external call it should take no more than 15 minutes for the call to successfully send data to SFDC. However, it is possible that you have your external calls delayed (to check, go to edit the external and view "Delay this call by" and you'll see the number of hours the call is delayed by) which can make it appear to slow down the queue. But it sounds like the behavior you're seeing is abnormal.
You can go to the integration status area>reporting>integration reporting and pause the queues then unpause them. Sometimes that can jump start things again. But even with doing that, I would suggest logging a ticket with support because a backlogged pending call queue means lead updates/creation isn't happening...all the stuff that has direct impact on revenue is affected. For us, the stuck pending calls was not anything on our side but because something funky was happening with Eloqua on the back-end that only support could address.