How to render a matte with Eevee? How can I render a matte (white on black or white on transparent) using Eevee ?

What I ask is a type of render that would render faster than a regular one. 
I just need the "shapes", not the materials / reflections / colors.

If it exists, I would like to be able to "switch" to the matte render without having to change every material.

Currently, what I do is that I disable all the lamps and objects I don't need, set the world to black color, and set film > alpha > transparent in the render panel.
It works but I guess that there might be a better solution.

Possible ?

Thanks !
If you've already set the World to 'Transparent' then you shouldn't need to disable the lamps or set the world to black - just use the Alpha channel of the output as your matte - it will be 1.0 wherever it hits an object and will be 0.0 wherever it does not. In Eevee, transparent objects will produce alpha of 1.0 (as transparency is only simulated in Eevee). 

For a faster render you could switch off unnecessary features (that don't affect the Alpha render) such as Screen Space Reflections, etc.