I guess I'm just thinking in terms of these tools as being text in, text out transformers. And it seems like an embarassingly parallel workload. So speed seems like the wrong thing to optimize to me.
Now we have large docs sites with thousands of pages, and it's painful if their static site takes forever to build, so we should also optimize for speed. Build-time speed reduces the feedback loop and it also saves developer time. It's like having a better computer somehow.
Docusaurus is also IO intensive as we read a lot of markdown files on the disk and also emit intermediate files.
Webpack already builds things in parallel, but it remains nodejs, and IO/CPU are both bottlenecks at different steps of our build.