The instances communicate between each other using special bots in the background that transport info between instances. Dot world is too big and makes too many requests to other instances. Most instances are reducing the number of transport bots back to dot world right now. There is supposed to be a fix in the next Lemmy version, but the code base is somewhat slow moving due to only two devs and it is written in Rust. Rust is a hard language like C, and not too many here are able to contribute to it, even though it is like the new gold standard of code.
So you might see a delay between posting and replies or the interaction may come in bursts that correspond with the transport bots carrying content between instances as the host admin have configured their instance.