Why fast liners make expensive armed merchant cruisers

What would make fast ocean liners suitable as armed merchant cruisers?

Speed is attractive, but high running costs and machinery optimised for fast passages make extended patrol work an awkward fit.

Commercial economy can reward rapid passage through higher freight charges and more productive voyages. A warship must travel to an operating area, patrol and return after an extended deployment. Those duties demand a different machinery operating pattern. Wartime escorts keeping pace with fast liners illustrate the strain of prolonged maximum revolutions. Liners could sustain such passages, while warships' machinery and crewing arrangements were intended for different use.