Namebench
While the app still runs in High Sierra, you won’t see any output. However, thanks to commenter Marc K., you can still see the output once it’s done. Open Terminal and paste this command:
find /var/ -iname "namebench_*.html" 2>/dev/null | xargs open
This will open the results page in your browser—including the below-referenced eye candy—so you can compare the various DNS that were analyzed. Thanks Marc!