Do you mean cruise away as in they activate their cruise engines? If they don't activate their cruise engines, then their lagging. Speed usually varies, could be anywhere to over 9000 m/s to simply just drifting through space at 1 m/s, doing nothing else.
You know, he has a point for once. This happened to me too while on the Dies.Irea. Took a mission, flew to it, battlecruisers flew away and out of sight and then occasionaly cruised in only to cruise away again. Was quite odd.