I assume you’re timing a short local road loop from the door for your comparison?
How exactly are you timing these rides?
There’s no way the mtb (assuming off road tyres) is as quick, it’s something you are (or are not) doing efficiently on the roadbike. Whether that be position, fit or something else.
No number of pics of you leaning against a fence is going to help you with this.
On the road you’re basically only fighting against wind resistance,gravity and finding your most efficient position.
Don’t worry though, if you stick with it long enough you’re sure to figure it out at some point.