well, this explains it:
That's an important qualifier. Convection ovens rely on a fan to circulate heated air throughout the oven — sort of a reverse wind chill effect. To compensate, you'll need to lower the temperature or shorten cooking time when using recipes designed for standard radiant ovens. Failing to reduce the temperature is a common mistake, one that may produce food that's burnt on the outside yet not cooked through.
most ovens automatically reduce the temperature when you turn on the convection mode, so it doesn't end up taking less time. i want to say it's like a 25 degree difference.. but don't hold me to that. anyway, you can cook more in the oven... like three racks of cookies at a time vs. one, so in that sense it would save you time.