yup pootling around with heavy tyres, heavy bike on trails or surfaces that are more efficient on a lighter built bike. Same for minor obstacles, that on a lighter bike may require some rider input, but on a burlier bike can just be steamrolled.
The same can be said about climbs, why waste the additional energy getting a bike to the top of the hill if you don’t need that much bike to get back down it and enjoy it.
A couple of mates take their MTBS out on road rides, I couldn’t imagine anything worse, prime example of being overbiked. a gravel or road bike will be faster, zipper and generally more fun in that situation while using less energy.
Now some of this might be moot depending on what you get out of riding. I ride for two reasons, the exercise and the technical challenge (it’s like a computer game to me), yes i want to get down the trail and get a PR, but if that was my primary driver i would be riding something like an Orbea Wild on super gnarly tyres everywhere.