I went riding a few weeks back with my co workers which all roade road bikes but at that time i didint have my road bike with me. I used my friends specialized hard tail bike. I was able to keep up and dropped majoirty of the riders on road bikes. I kept up and paced the leader the whole time.

I keep tucking and getting into an aero position on the mtb, something like invisible handles... we were riding about 28mph +/- on a good pull.