KFIS - the brain is a big bundle of highly interconnected neurons. Communication between neurons in the brain involves similar processes to those involved in communication between neurons in the peripheral nervous system. Signals (action potentials) travel along long projections (axons) from the cell bodies. These projections form connections (synapses) with other neurons. At these synapses the arrival of an action potential causes chemicals (neurotransmitters) to be released into the gap between the two cells. These either stimulate or inhibit the generation of a new action potential in the post-synaptic neuron. So the speed at which action potentials propagate, and the speed of the chemical reactions that occur at synapses, are both constraints on the "speed of thought" assuming you accept that neuronal activity = thought.