nicco.io/readingTime.js

import getReadingTime from 'reading-time'
import { toString } from 'mdast-util-to-string'

export function remarkReadingTime() {
  return function (tree, { data }) {
    const textOnPage = toString(tree)
    const readingTime = getReadingTime(textOnPage)
    data.astro.frontmatter.readingTime = readingTime
  }
}