When a website has performance problems, the usual response is to optimize what is already there.

Minify CSS.
Delay JavaScript.
Remove unused styles.
Compress assets.
Improve the PageSpeed score.

But there is a more basic question:

Why deliver something that a device does not need in the first place?

A phone does not necessarily need the same frontend resources as a desktop. Yet conventional responsive web design often sends the same CSS and JavaScript to every device and lets the browser decide which parts are actually used.

That works. But it also means we may spend time optimizing resources that never needed to be delivered.

There is an older concept called RESS - Responsive Design + Server-Side Components.

The idea is simple:

Desktop → desktop.css
Tablet → tablet.css
Phone → mobile.css

The server decides which resource set is appropriate before the page reaches the browser.

And this does not mean abandoning responsive web design. Each device-specific stylesheet can still use normal media queries and responsive techniques.

RESS simply adds an earlier decision:

First decide what the device actually needs. Then optimize what remains.

Maybe "performance optimized" should therefore mean more than minification, caching and delayed loading.

Maybe users should also start asking theme and plugin developers:

Does your solution avoid delivering frontend resources that the device will never need?

EDIT: Judging from some comments, my CSS example may have been too narrow. RESS is not about the server replacing media queries. I made a small demo showing the actual idea: the server delivers different PHP, HTML and assets depending on the device, while responsive design can still be used normally inside each version.

Demo: https://www.imedes.dev/ress/