Yes, the liner days were all about speed. QM2 was built with speed in mind, but also partly compromised by the need to be a good cruise ship too. She likely has the fastest top speed of any current passenger ship, but doesn't use it - not because of comfort, but fuel economy. 10 years or so back when the price of fuel shot up, they added a day to each of their Transatlantic voyages, so they were no longer trying to get from Southampton to New York as quick as possible, which is against the ethos of a line voyage. Even at speed though, QM2 is quite comfortable.
Yes, a cruise ship is generally built for comfort, and will usually slow down when conditions aren't favourable. On one of our Noumea to Sydney crossings, Radiance was going 5 knots faster than we have travelled on QM2. The current must have been good that day, as we were going faster than the ship specs say for her top speed. It was a little less comfortable, but a fun afternoon to soak it up on the promenade deck.