I have a very comfortable ~500€ basic mountain bike with 2.2" tyres and 27.5" wheels on which I can maintain an average of 24-25 km/h on a windless day on flat asphalt when I'm feeling strong. I cannot keep pedalling in top gear continuously. Instead, I keep switching between the hardest and next to hardest gears.
How much easier would it be to go fast on an affordable road bike with narrow tyres? Sure it has "faster" gearing, but that alone won't help me since I don't spin out on my current bike either. I am simply not strong enough. Would I see a real speed improvement from a road bike, and if yes, approximately how much? Does that improvement come mostly from a more aerodynamic (but perhaps uncomfortable) position, less rolling resistance, better position for pedalling, or something else? I assume all of these matter, but there is likely a dominant factor.
I have never had the chance to ride a road bike, and I am simply curious. No, this question does not stem from any practical need such as commuting. However, it would be nice to know if a more efficient bike would allow me to reach more places within a reasonable amount of time during this pandemic when I am hesitant to take public transport. I suspect it wouldn't (without better fitness).
EDIT: Another way to phrase this question is, does it even make sense to think about a more efficient bike until I regularly spin out on my current one?