What structure ensures a slow flow of the lymph nodes?

Lymph enters the lymph node through a series of afferent lymphatic vessels and exits the lymph node via efferent lymphatic vessels. There are more afferent lymphatic vessels than there are efferent lymphatic vessels, slowing down the flow.

The reason the slow flow is desirable is that it allows time for lymphocytes and macrophages to carry out their protective functions of removing debris from lymph.