I'm not sure that the problem is heat itself or roots cooking.
the problem with rocks and slabs is that they absorb water and evaporate it into the air. That means your pot needs much more water than an equivalent tree without the rock. There is also the fact that the rock takes up some of the space in the pot so there is less potting mix and therefore space for water in the first place.To keep bonsai with rocks and bonsai on slab healthy give them more shelter than other equivalent sized pots and water more often.
