It's nothing like 15 minutes, but Lemmy doesn't federate posts instantly either. At a guess, there's a per-remote-instance worker that sleeps for a bit, then sends everything that's accumulated while it was sleeping. It's most noticeable when you're linked to only one other instance, and you still have to wait before getting anything. The advantages are that it's better to open a network connection, send a bunch of stuff, then close it, rather than opening and closing it for every activity, and it's more efficient to just send an Edit, rather than a Create and then an Edit if they both occurred close to one another.

For Threads, there's the additional advantage in that it means they can offer the equivalent of 'undo send' (like in Gmail), since deleting a non-federated post is easier and more reliable than deleting a federated one. But 15 minutes is crazy high - like the Source says, it makes a nonsense out of trying to do things like comment on a live event.

(In contrast to the above, PieFed will send this Note out instantly. It's all a trade-off between the pros and cons of different approaches, innit)