Debugging rebuild performance

Our app is using ember-cli-postcss and on rebuilds the PostCSS step is always the slowest node even though no CSS files have been changed:

file changed templates/components/koenig-card-embed.hbs

Build successful (9845ms) – Serving on http://localhost:4200/

Slowest Nodes (totalTime => 5% )              | Total (avg)         
PostcssCompiler (2)                           | 5637ms (2818 ms)    
Vendor JS (1)                                 | 2704ms

My assumption is that it’s something to do with the build output not being cached properly, does that sound likely? What would be the steps to debug a build issue like this one?