I have had road bikes since the mid 1960's. So when I returned to cycling [again] after retiring I naturally bought a road bike. Last fall I found and bought a used mountain bike. I thought it might be handy to extend my cycling season into winter. I can't say my experiment was a complete success.... but I did get in a few more miles and well as a bit of fun with the Mtn bike.

I also found average speeds weren't exceedingly different. The mountain bike has about 15 pounds on my primary road bike.... so the big hill near my home should take a few seconds longer to climb. But descending the hill seems pretty much the same. The lighter wheels and tires on the road bike accelerate much faster than does the big chunky tires and heavy wheels on my mountain bike. So the road bike saves a few seconds at every stop-start.

But since I didn't try any L O N G rides on the mountain bike (or any real hilly areas)... my average speeds aren't too different.