Final answer:
Increased norepinephrine primarily speeds heart rate in the sympathetic nervous system.
Step-by-step explanation:
The primary factor that speeds heart rate in the sympathetic nervous system is increased norepinephrine. Norepinephrine is released by the sympathetic nerves and binds to adrenergic receptors in the heart, causing the heart rate to increase. This response is part of the body's fight-or-flight response, preparing the body for physical activity or in response to strong emotions.