The most obvious questions:
What language are they using to run ExtractBot?
How did they identify the expensive function?
Where was this expensive function (CSS bot is mentioned, is this their code or did they use a lib in which the fix would be of interest to others)?
Is the ExtractBot home page demo form purposefully broken at present due to HN load, or just broken for me?
Without knowing those things, some guesses: It's a scraper in Node or Ruby, it uses a load of existing libs that were not written with performance in mind. Those libs pull apart HTML and extract text values which are returned in a JSON doc (or something). They wondered why they were running so hot, managed (intentionally? fortuitously?) to spot some loop that needn't exist, or an expensive function that could be avoided by some cheap caching.