There's a small loss possible due to tires slipping a bit, also higher rolling resistance due to usually wider and less smooth tires. The grade needs to get steeper than 10% on pavement or off for the weight penalty of a bicycle to count against the rider versus the efficiency, depending upon the weight of the bicycle+rider. If you look at the times of guys in the Tour de France for climbs up mountains the fastest can do a four minute mile on the uphill parts, here's last years winner on a 10 mile climb https://www.bicycling.com/tour-de-france/a44465449/tour-de-f... For reference if a marathon runner could go that fast I believe they would run a 1:45 marathon - up a mountain. The top end cross country mountain bikes weigh about 6-10 pounds more than the bikes the road professionals are using.