Learn Spanish (Spain): El papeleo es lento, puesto que hay pocos funcionarios. - The paperwork is slow, given that there are few civil servants.

Causal connectors like puesto que, ya que, and en vista de que introduce a known fact or an objective reality that explains the main clause. Because you are declaring an established fact (there truly are few civil servants), you must use the indicative mood.

Can I just use porque instead of puesto que here?

You could, but puesto que (given that / since) elevates the register. It is a bit more formal than porque and is frequently used in written Spanish or news when explaining the logical cause of a situation, especially when the reason is already somewhat known to the listener.

Does funcionario just mean any functionary or office worker?

In Spain, funcionario has a very specific meaning: a civil servant or public government employee. It implies someone who works for the state, regional, or local government, usually having passed strict exams to get a permanent job. It is never used for private sector office workers.

Why use es lento instead of está lento if we might be talking about a specific delay?

Using es lento describes an inherent characteristic: the paperwork or bureaucracy itself is naturally slow. If you used está lento, it would imply a temporary state, meaning the system is usually fast but happens to be running unusually slowly at the moment.