Why CW courtesy says you answer at a speed the other operator has already shown they can handle, and the speed they sent CQ at is proof of that. So you match their speed or go a bit slower, never faster, and never faster than you can comfortably copy yourself, since they will likely reply at the speed you called with. Calling faster than you can receive just means you cannot copy the answer you get.
Watch out The choice saying 'no slower than the CQ' has it backwards, forcing you to keep up with a speed you may not be able to copy. There is no FCC or convention-mandated standard calling speed of 10 or 5 wpm.
Answer a CQ at the sender's speed or slower, never faster. You will get back what you send.