I found Wellington, a Sass compiler written in Go.
I installed Wellington with Homebrew - actually Linuxbrew but that's a post for another day maybe, once I'm sure this Linuxbrew experiment worked for me.
$ brew install wellington
This is not the night to redesign the whole site, though. Make sure everything works.
$ wt compile assets/scss/main.scss -b static/css 2017/10/31 21:09:54 Compilation took: 28.333622ms
Seems to produce the same style output. I had no complaint about the speed of Ruby's Sass compiler, but Wellington is certainly quicker.
I guess now I can start thinking about redesigning the site layout.