I also suffered this slowness on gulp and found out it was cssnano the big responsible of it. Although I know it’s deprecated, I switched again to minify-css and difference was enormous, from 9s to 1.8s when compiling styles. That’s a big change if working on small changes.
Browsing for a solution to this problem I found this benchmark study: http://goalsmashers.github.io/css-minification-benchmark/
Is there any reason to change from gulp-cssnano in favour of gulp-clean-css, which is recommended by gulp-minify-css npm repo?
Using gulp-cssnano is killing my workflow. 