I have some data on this topic, having used a power meter for a few years, and tracking things carefully up until about 3 years ago. At the same power, speed difference #1 is aero. Not just an aero bike/position vs upright, but also clothing, helmet, jacket, they all add up to slow you down. Difference #2 is wheel/tire/pressure. Aero is part of that equation, but just as big is quality of tire (casing) and pressure. Most people ride with too much pressure for the road conditions. Next time you notice the speed difference as you transition from a chip seal road to a smooth one you get an enhanced example of what difference tire quality and pressure can make. That speed difference I can prove by going from cheap tubulars to high quality tubeless clincher at low pressure. Difference #3 is body position and comfort. A coach at an indoor class I took a few years ago pointed out that when racers switched from a road bike to a tt/tri bike (indoors) he would typically see a 5-10% average power drop off because they had not adapted to the tt bike well. He looked at my data and noted that I did not happen to have that dropoff. But he also mentioned that switching bikes could amount to the same difference. I have no idea how much stiffness matters because any bike I had a power meter on was modern.