I definitely agree that a fast start is ideal in most cases, but just to add something as an example of a slow start that was justified: in SM, if you'd gotten your starter before rather than after meeting Lillie on the bridge, the bridge scene would have gone down completely differently, which may have been something that Game Freak wanted to avoid or that would have kept them from telling the story they wanted. Something like that - where your game has a very strong beginning as it is and you can tell a better story by waiting to give Pokémon, whether to show that the player is already a good person without relying on them or because having Pokémon would change the resolution of a problem - is definitely a valid reason to hold off.
But if it's just exposition and not things happening in the present and to the player, you can surely stand to wait to do those things until the player has a Pokémon! Do whichever gives you more options, not fewer.
In before USUM have the same plot, but you get your starter sooner and the entire thing snowballs from there into a different story.