For example, postcss-scss is not a plugin but a parser, which does not compile scss to css. I know there is a gulp-sass and it works just fine.
The reason I would like to start using a plugin for postcss is because they say postcss can be more than 30 times faster.
But it seems I can't find a plugin for postcss which would compile scss to css. Please, help.
Be aware that PostCSS shouldn't be used together with SCSS in one workflow routine as this pretty much defeats its purpose. It's either SCSS to CSS or PostCSS to CSS.
As you already stated, there is a postcss-scss parser, but that serves the purpose of using SCSS syntax when working with PostCSS:
PostCSS can transform styles in any syntax, not just CSS.