I have a MTB with a suspension fork. No rear suspension. I put some street tires on and noticed a big difference over the knobby MTB tires as far as the rolling resistance. I can maintain a speed with easier effort with the street tires on. See if you can tighten the rear suspension up to reduce the lost work due to the compression of the suspension. I also use the lockout, or close to it, on the street on the front shocks to reduce the lost work even more. I suppose the other option is to buy the hardtail MTB and slap some road tires on there. That should net you the biggest gains in the rolling resistance and work put into pedaling I think.