Mountain bikes, gravel bikes, are slow
Given the huge difference between the 2 I'm not sure what you mean? Also because these days 'mountain bike' is an umbrella term, whereas gravel bike is still rather specific. E.g. that last link shows already a rather specific segment of maountain bikes: dual suspension, quite a lot of travel on the front suspension, totally suited for going down the hill fast as well.
Very similar to cycle cross (a wide range of gears) https://upload.wikimedia.org/wikipedia/commons/thumb/c/c7/Fo...
The hybrid has straight handlebars https://upload.wikimedia.org/wikipedia/commons/thumb/2/22/AE...
Mountain bikes, gravel bikes, are slow. Gearing is for climbing a mountain. https://upload.wikimedia.org/wikipedia/commons/thumb/1/1f/No...