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Why is saving a BootstrapTheme so slow?

Short answer: it is difficult to add vendor-specific CSS property prefixes without using Node.

Bootstrap uses vendor-specific prefixes to add support for modern CSS features in a wider variety of browsers. Flexbox requires vendor-specific prefixes for IE10, for example. The Bootstrap SCSS code does not include any vendor-specific prefixes because they are added after-the-fact with an autoprefixer Node command. Because this plugin uses Python to compile SCSS into CSS, the compiled CSS doesn't include vendor-specific prefixes.

My solution was to use cssutils to parse Bootstrap's compiled CSS and extract only those CSS properties that used vendor-specific prefixes, then, using cssutils again, splice these properties into the libsass-compiled CSS. This process takes around 5 seconds on my AMD Ryzen 5 1600.

If you can think of a faster way to accomplish this, please open an issue and let me know!