Because the JSON grammar is much simpler than
    JavaScript’s grammar, JSON can be parsed more
    efficiently than JavaScript.

Hmm.. shouldn't that hold for most programming languages then?

Let's try it for PHP:

    time php -r 'for ($i=0;$i<10000000; $i++) $data = [1,2,3];'
    
    real 0m0,173s
    user 0m0,161s
    sys  0m0,012s

    time php -r 'for ($i=0;$i<10000000; $i++) $data = json_decode("[1,2,3]");'
    
    real 0m4,125s
    user 0m4,120s
    sys  0m0,005s

So for 10 million repetitions, a small PHP structure is about 20x faster then parsing JSON. But to test the point of the article, one should use the sama data structure it uses (https://raw.githubusercontent.com/WebKit/webkit/ffdd2799d323...) and parse it only once.