I just came into a like new Bacchetta Giro 20 (steel with the heavy recurve seat) after not riding all winter. The previous two years I kept track and measured my regular routes on a Novara Randonee and Raleigh Clubman, neither being a speed demon, but nor am I.
Don't laugh at my averages, but on the uprights for the previous two years, I've seen overall numbers from 11.5-14.0 mph. This includes long red lights and traffic stops, since I don't hit the pause key on the app I use on my phone unless it's a long break. These routes vary from 25-50 miles with some hills.
I've recently run the very same routes on this Giro, clearly without bent legs. I'm seeing overall times almost the same, maybe 1 mph slower overall, especially with a few hills. Let's say I averaged 12.5 on the touring bikes, now 11.5 on the Giro. Does this 1.0 mph gap equal just 10 extra minutes on a 25 mile ride, and 20 minutes on a 50? Just curious about my math. The odd part is I feel slower, but the gps shows otherwise.
I guess my main observation as primarily an upright rider, is that you might think you've taken a big step back in overall speeds, while the gps says otherwise. All numbers aside, the comfort and view are worth it, although I can see where the knees can take a beating.