Hi Kevin,
We host a lot of webinars and sometimes face the same challenge. For us, registrations are usually passed in 60 minutes or less, but periodically we see very long lags of 3-5 hours. We have tried many different configurations, some at the suggestion of tech support and some from topliners, similar to what Michael McKinnon mentioned above.
We haven't found a solution that is consistently superior to another... I say this knowing that everyone's experience has been a little different. From what I've learned the delay or processing time isn't always in the Eloqua system, but can also be on the Citrix side.
To get around the delay we've made it common practice to send our "last chance" invitations the afternoon before the event, instead of just hours prior. If we want to do an "extremely last minute" invitation, we simply use the registration link that GoToWebinar provides.