It seems like a quick repentance: but how much did the Ethiopian already know about the God of Israel? How long did Philip ride along with him and how much could he tell him about Jesus? This quick
repentance and baptism even brings today’s interpreters to some criticism on the established church
which places the bar too high with education and entry courses before adult baptism can (finally) be administered.
But then you miss the uniqueness of this history. This story is unique and not meant to be directly followed as a prescription. What and how the Spirit acted at that time was especially necessary in the time of transition. He can always provide such miracles, but that is something different than that we should expect a repeat of this event as the normal way the Spirit works. The directions from the angel and the Spirit in Acts 8 also remain unique at that special time that the gospel was crossing borders. God can also do that today, but he has not promised us to do it in that way.
We also need to remember that the Ethiopian was a God-fearing person. He could have known the Scriptures very well, but still needed to understand that all of Scripture points to Christ. And this is what Philip came to show him.
38 And he commanded the chariot to stop, and they both went down into the water, Philip and the eunuch, and he baptized him.