So, this material would take a long time to heat up, but once it heated up it would stay hot for awhile?
Rather than just saying it would take a long time to heat up, it might be useful to compare it to other well known materials. Stainless steel for instance, would heat up 5.4 times faster than Cordierite per the same amount of mass, but Cordierite has the capacity to hold 2.93 times more heat than stainless steel per the same amount of mass. Another ceramic, Alumina (Aluminum oxide), will heat up 11 times faster, but hold 1.94 times less heat. By comparison, Cordierite takes longer to heat up than these materials, but you have to consider that these materials transfer heat so quickly, you could end up burning the bottom of the pizza.
Based on its thermal properties, I don't see why it would take much longer than a normal pizza stone if it's of similar size and weight. If you tell me what temperature you want to bake at, I can tell you about how long it will take to heat up.