In any boat where the mainsail is a significant portion of the sailpower, the traveller can in some ways be looked upon as the gas pedal.
When everything "looks" OK, sail shape, draft, and the boat feels powered up but boat speed is off, easing the traveller to leeward (down) a few inches will often result in an immediate improvement in speed and easing of weather helm, which, if excessive, also hurts boatspeed.
This effect is very dramatic on fractional rigs
, less so sometimes on large "J" masthead boats.
In heavy air easing the traveller reduces the angle of attack of the sail and reduces the likelihood of stalling. It is not unusual, nor necessarily slow, to carry a slight backwind bubble along the luff of the main in those conditions.