Hour lag in getting ContactID from Visitor Lookup using GUID with net new contacts

In the Eloqua API you can do a Visitor Lookup using the GUID (doc here). The great thing about this is it’ll return a ContactID (if one exists) that I can exchange for lead field data. However, what I’m finding is that this request will be empty for a new lead for up to an hour. So my first question is: Why? And is there anything I can do about it?

If I use the web data lookup with the same GUID, I get back lead data right away. But what I don’t get is a ContactID. What is needed in a data lookup to get the ContactID, and is it possible to get it from a lookup using the GUID and not the email address?