I can confirm that the soapstone stoves take longer to heat-up. Based on my experience they are a little harder to overfire as well. Get a lazer thermometer to check you stove temps and don't reload above 400 and you should be fine. Do you have a blocking plate in your chimney? They really help the stove efficiency in terms of heating the space.
By blocking plate do you mean blocking the damper space where the stove pipe runs through? If so, I don't have a plate, but I've stuffed it with insulation to keep less air from escaping into the chimney.