If you were talking MTB in Offroad MTB trim and doing the MTB bit offroad- then you could say that the MTB is 95% harder than the same distance of a road bike on the road.

I used to ride Mountain bikes solely and may change the tyres for a metric century on the road- But I have a 40 miler that I do. On the road bike I do it in 2 1/2 hours. On the mountain bike in offroad trim-I do it in 2 1/2 hours.

It has a few slopes in it but one section with plenty of sharp corners and 5 miles at the end with plenty of traffic. It's just that the MTB suits the conditions better. All I know is that the MTB is hard work and the road bike is easier.