[bbulkow]

Possibly neither, that is, there are many possible reasons.

for example, stateful firewalls can only track a certain number of connections. If you don't have a public IP, or if they are trying to protect you, it may be necessary to wait for a connection resource to come available. Connections are generally expensive, different from data transfer, for these and other reasons, such as port starvation, syn flood defenses inappropriately targeting you, so many others.