Real-time Leaderboards: Fan-out versus Fan-in?

hey all, so im building a new feature for a game, it needs a leaderboard that updates instantly. we think lots of people will be writing scores (game events) and reading them (players checking how they rank). im kinda stuck between two ways to do this: fan-out, where you write to lots of places and read from one, and fan-in, which is the opposite write to one, read from many. fan-out sounds great for reading, but writing could become a huge problem if we have millions of players updating their scores all the time. fan-in might make writing easier, but then reading could get jammed up. how have you guys handled real-time leaderboards? did you go fan-out or fan-in, and what made you choose that? anything else I should be thinking about to make this scale right