Here's what I experienced when the Texas Eagle was detoured. They adjusted the schedule from the point where they left the scheduled route (St. Louis) to make the full time (at 59 MPH) between St. Louis and Chicago with NO padding then added to the end. Since there is already considerable padding on the regular schedule, it APPEARED that the TE would arrive EARLY because of the shorter distance, no stops, etc. In reality, we departed St. Louis an hour late and never regained that time back and thus arrived into Chicago an hour after the revised time. And I don't remember there being any slowdowns or passings the entire way.
So my answer to the OP is that though the route may be shorter, all the regular traffic will be detoured to the same line AND the detoured route may not have an increased "P" speed limit because it doesn't normally carry passenger trains. That means that instead of 79 MPH, you're likely to plod along at 59. Such was the case on the TE detour. Come to think of it, I think the time keepers on the TE reroute a year ago June probably calculated the time at 79 MPH but kept losing time along the route even though we didn't stop and we seemed to be going full track speed the whole way.
Meanwhile, enjoy the rare mileage!