What do you mean the reflex arc is very long?

To put it bluntly, it is "metaphor", which means that you are slow to respond. (Maybe it's a joke)

It can be understood visually that A needs to go through X to big B. The longer X, the longer it takes for A to get to B. That is to say, you are slow.

Reflex arc is a nerve in the human brain. The person who said this means that you are too slow. For example, if he tells a joke, others laugh and you laugh, which can be said to be a long reflex arc.

Further reading

The reflex process is simply: a stimulus is felt by a receptor, and the receptor produces an exciting reflex arc (if the body is damaged, the body has neither feeling nor influence); Excitement is transmitted to the center through afferent nerves in the form of nerve impulses (if the body is damaged, the body has neither feeling nor influence); Through the analysis and comprehensive activities of the center, the center produces excitement (if the body is damaged, the body has neither feeling nor effect); The excitement of the center reaches the effector through certain efferent nerves, so that it can make corresponding regular activities to external stimuli according to the excitement from the nerve center (if the damaged body just can't feel the influence); Excitement is transmitted from the nerve center to the effector (if the body is damaged, the body only feels and has no effect). If the center is suppressed, the original efferent impulse of the center will be weakened or stopped. Under experimental conditions, artificial stimulation directly acting on afferent nerve can also cause reflex activity, but under natural conditions, reflex activity generally needs to be completed through a complete reflex arc. If any link in the reflex arc is interrupted, reflex cannot occur.