A reflex is faster than a voluntary reaction because of the relative complexity of the neural pathway of reaction compared to that of a reflex, according to Carolina Biological Supply. Each type of response is initiated by a sensory stimulus.
In a reflex response, the stimulus impulse travels only to the spinal column and back to a motor neuron which triggers a muscular action. This all happens on the same side of the body. Voluntary reaction requires the stimulus impulse to travel to the brain, then a response is sent back through the spinal column to the muscles. BioTopics points out that this process involves more cells and synapses, as well as a longer path up and down the spinal cord.