No the slope is calculated per pixel, not per vertex, which is what makes it incredibly expensive for what it provides.
Moreover, the calc is happening everywhere at all times, even if the pixel is already rendered out - which is even more expensive.
On top of that, then, you also have the cost of the different layers - but you probably have the cost of the layers anyway when you paint them with identical slope maps.
The cost reduction is not having the shader continuously processing things it doesn’t really need to process…