It works using Chrome's remote debugging API, so it should be more accurate than a DOM simulation. It accepts arguments on the command-line for configuring things like device scale factor, width, height, and user agent.
It also has an API, which is ideal for dropping it into a build script.
If RWDPerf indicates that your page has a lot of unused images, then you might want a better responsive image replacement library. Respimage (GitHub: aFarkas / respimage, License: MIT) by Alexander Farkas is a responsive image polyfill for
Respimage works using the
picture element or the
srcset image attribute. The
srcset implementation supports descriptors for width and density.