How much easier is it to go fast on a road bike and why?
Aerodynamics are very important in cycling, and the faster you go, the bigger a factor aerodynamic drag becomes, because it increases with the square cube of your speed, but other sources of drag (mechanical resistance, rolling resistance) increase linearly. So putting your body in a more aerodynamic position is important, and not something you can really replicate on a mountain bike. At the speed you're going, aerodynamics are not trivial, but not overwhelmingly important.
Rolling resistance is the second-biggest source of resistance. You could put high-quality slick tires on your bike and get faster right now.
If you are commuting in a city, your travel time has more to do with the number of stoplights you hit than your top speed. If you race from one stoplight to the next and then spend more time at the stop, you won't reach your destination much faster. Your &quot;light-to-light&quot; time would need to improve enough that you would beat the stoplight.
&quot;does it even make sense to think about a more efficient bike until I regularly spin out on my current one?&quot; That's not a good way to look at it. Bikes will generally have top gears that you'll never use except on a downhill or with a tailwind. If you regularly spin out in your top gear on level ground in still air, your top gear is too low for your riding conditions.
You would probably be faster on a road bike, or you'd ride the same speed for less effort. How much faster is hard to say. 25 km/h is a pretty good speed for urban riding.