Once upon a time, I did a long climb twice: once on a road bike, then on a mountain bike. I kept my heart rate in the same range to maintain the same level of effort, and I did the two climbs on different days. The mountain bike was 15% slower, even though the MT bike+rider mass was only 3% more than the road bike+rider mass.

FYI, the route was Big Pine, CA to the Schulman Grove. The profile:



Mountain bikes are slow.