Yes I do ride an MTB, but it is a reasonably light Cannondale, with 1.25" slicks pumped to 100psi. My top gear is about 104 gear inches. I'm not sure I'd be much faster on a road bike, although I'd love to try.
I also have a Cannondale MTB with 1.5" slicks, no suspension, and it is <10% slower than my road bike, so I will usually ride it on longer rides because it is a bit more comfortable, takes more of a beating on bumpy country roads and is less susceptible to flats. I just love this bike, it is a large size frame so I can ride in a more stretched out position than an upright position.
My next road bike will be a Cannondale also, just have to convince Anna that a bike is more important than fencing, landscaping, pergola etc.
CHEERS.
Mark