Based purely on the article content, it appears to be "less time to author":

"CSS Grid has made my layout process faster and simpler, with more flexibility. We can get started with a few basics, and the fallbacks don’t have to be overwhelming"

I assume the former. From the article:

"Don’t let that stop you: CSS Grid has made my layout process faster and simpler, with more flexibility. "

Grids and flexbox are on par with CSS tables for layout performance. Slow as hell to reflow.