Learn Spanish (Spain): El pago en efectivo es más rápido.

Yes, efectivo often means "effective" as an adjective. However, the set phrase en efectivo is the standard way to say "in cash" in Spanish. In Spain, you might also hear the phrase en metálico used to mean exactly the same thing.

Could I use the verb "pagar" instead of the noun "el pago" here?

Yes, you could absolutely say pagar en efectivo es más rápido (paying in cash is faster). In Spanish, using an infinitive verb like pagar as the subject of a sentence is the normal way to talk about the act of doing something. This sentence simply uses the noun el pago (the payment) instead.

Why do we use "es" instead of "está" if being fast at a checkout feels like a temporary situation?

Even though a specific transaction is over quickly, we use es (from the verb ser) because we are stating a general, universal characteristic of cash payments compared to other methods. We are classifying this payment method as a "fast" one, which describes its core nature rather than a temporary condition.

Is "rápido" acting as an adjective or an adverb in this sentence?

It is an adjective here, describing the masculine singular noun el pago. Because it is an adjective, it must agree with the noun it modifies. If you were talking about a feminine noun, like a bank transfer (una transferencia), the adjective would change to match: una transferencia es más rápida.