I am currently riding an entry level carbon road bike (Fuji Gran Fondo 2.5, compact 50/34 crankset, 105 group, 12/30 10 speed, Oval 327 wheels, but I suspect the details are not important-you have all seen similar bikes on club rides) My typical ride is 30 miles and 1500 feet of climbing (as reported by Strava, if I add up the hills they are not this high) in 2 hours, so I am not a speed burner. The Gran Fondo is promoted as an "old guy's" (which I am) bike-the head tube is a bit longer (only 1/2 inch) than standard so you sit more upright, you get (my year, not now) the 30 tooth on the rear and the tires are 25 instead of 23. Will another bike blow me away if I ride it a short distance (5 miles) on the flat? How much time can I save at the same power input over the 30 mile ride? Let's pretend I have no budget limitation, I just want the fastest (easily available commercially) road bike for my use. How much faster is it than what I have? What would you look for? Don't send me looking for an 11 tooth on the cassette-I rarely use the 50/12. You need to send me to a class descending hills faster first.
Added based on Craig Hicks answer: I have inverted the stem, which probably counteracts the longer head tube of the bike I have. I didn't notice any difference in comfort. I could take a couple spacers out of the the head tube to lower the bars another inch. I did a high end fit with my LBS with some comfort improvement but no speed. I suspect the biggest speed increase I could buy would be race tires that have less rubber so less flexure loss on the road at a price of durability. That is what I was looking for, what should I look for and how much would it speed me up?