Most cruise ships have a top speed of 23 knots but they tend to cruise between 14-17 knots.
QM2 has a top speed of 34 knots and when we were on her (Fremantle to Capetown) it was generally around 25 knots and very comfortable.
While most of what you said is correct , the QM 2 has a totally different hull structure to cruise ships and their cruising speed and comfortability reflect this.