Why and how?
Conscious processes are slower than unconscious ones due to the increase amount of neural activity required for conscious thought to action.
"Reflexes require a minimum of two neurons, an sensory neuron (input) and a motor neuron (output) (see Figure 1) The sensory neuron (such as a pain receptor in the skin) detects the stimuli and sends a signal towards the CNS. This sensory neuron synapses with a motor neuron which innervates the effector tissue (such as skeletal muscle to pull away from the painful stimuli). This type of reflex is the "withdrawal" reflex and is monosynaptic, meaning only one synapse has to be crossed between the sensory neuron and the motor neuron. It is the simplest reflex arc and the integration center is the synapse itself. Polysynaptic reflexes are more complex and more common. They involve interneurons which are found in the CNS. More complex reflexes may have their integration center in the spinal cord, in the brainstem, or in the cerebrum where conscious thoughts are initiated.
Many people conider only the simplest types of responses as "reflexes", those that are always identical and do not allow conscious actions. We must not confuse these with "reactions", which are different from reflexes in that they are voluntary responses to a stimulus from the environment. For example, while the body has various subconscious physiological responses to mitigate cold, as humans we can simply choose to put on more clothes. This is a conscious order made by the cerebrum, not an involuntary response to a stimulus. This is a very complex response involving millions of neurons and some time to process the voluntary response. In contrast, spinal reflexes occur much faster, not only because they involve fewer neurons, but also becuase the electrical signal does not have to travel to the brain and back. Spinal reflexes only travel to the spinal cord and back which is a much shorter distance. Because of this and the complexity of conscious reactions, they take more time to complete than a reflex. On average, humans have a reaction time of 0.25 seconds to a visual stimulus, 0.17 for an audio stimulus, and 0.15 seconds for a touch stimulus (2). Reaction times vary from individual to individual. Because of the higher degree of neural processing, reaction times can be influenced by a variety of factors. Reaction times can decrease with practice, as often times athletes have faster reaction times than non-athletes. Sleepiness, emotional distress, or consumption of alcohol can also impact reaction time."