cross country than lots black diamond downhilling so his bike is a hard tail.

Full suspension bikes are heavy but I think you overestimate the rolling resistance based on tire size.

I agree if you have exactly equal climbing with a bike that ways 2x as much that it’s going to be harder. It’s also not a given that a MTB ride is going to have anywhere near as much climbing as a road bike ride. My Saturday ride had 3x the climbing of my son’s 90 minute practice ride with the team on Monday.