Why to design for mobile and scale to desktop with media query?

Question

In the W3schools tutorial on CSS3 media queries they recommend to have the standard layout adapted to mobile viewing, and have a media query "scale it up" to desktop format when viewing on a desktop instead of the other way around. This should make the page display faster on mobile devices.

What is the reasons that it will display faster?

The obvious one would be that the mobile devices have less computing power and shouldn't have to do the media query and change the layout -- but can that really put some tension on the mobile's processor that is worth considering?

Answer 1

The real issue is that a mobile device has much less space for displaying than a desktop. While upscaling few information is easy, it's almost impossible to downscale large information in a tidy manner.

In other words, I would say the issue is not rendering speed, as much as a question of design practicalities.