I think what you're referring to is that querySelector is O(n), while the older getElement APIs are O(1).

https://stackoverflow.com/questions/14377590/queryselector-a...

That makes sense, but it has nothing to do with "string parsing", as the OP said. Parsing the query is fast. Executing the query is slow, because it's a more general API.

(And this is why the citation is important: because it shows the real issue, not the one described! Thanks for the clarification.)