It was actually after the sea trials, with a nearly full complement of 'shakedown' passengers, and while Lorna Dallas sang in the new Grand Lounge - it was quite something - and the vibrations shook the coloured filters off some of the spotlights above! Taking shakedown a little literally i think!
I don't have BP's book here, but his speed trials figure was about a knot slower than that... maybe they adjusted something after the trials, and wanted to check it 'on the way home'.