From your first pic, I thought that maybe your saddle was a little high, but watching the video, it now looks like maybe your saddle is a little low. It's hard to tell from the video and it depends on riding technique such as how much you drop your heel. How much did you lower it?
It would surprise me if your mountain bike is actually faster, If you have a bike computer, it's easy to tell, otherwise maybe time yourself on a known course. I suspect that the mountain bike just subjectively FEELS faster because you like it better.
Either way, ride what you enjoy. And you can put road bike tires on your mountain bike and call it your "other" comfort cruiser.