Quick response is the mode for questions that don't need long thought. You find it in the mode selector under the message box in Copilot Chat, next to Think Deeper. It answers straight away, which suits the syntax of a formula, a one-line message or a quick rewrite. Left on Auto, Copilot picks between Quick response and Think Deeper for you, based on the question.
Whichever mode answers, Copilot often grounds its reply in web results with citations, so check the sources before you repeat anything time-sensitive. If a quick answer falls short, ask the same question again on Think Deeper: it costs you about ten seconds more. When the task is a real research or data job, neither mode fits and you open Researcher or Analyst instead.