They will definitely extend the amount of time it takes for the cooker to come up to temp, as all that mass needs extra time to heat up. Did you open the vents any extra to compensate, or just put them in at 300 and waited ? Seems in the past (with the setter on the egg, haven't tried with the D's) if I added the ceramic after the fire was going, I opened it up for a few minutes to get the fire really hot, then stopped right before I hit temp.

Note: I usually add them when I light the coals, both vents wide open til I get about 25 degrees below target, then adjust accordingly.