It might not just be lemmy.world, but several times recently I've made posts (usually to !helldivers2.lemmy.ca) that I check throughout the day. Then like 8 hours after I've made the post, I'll see comments appear from people on lemmy.world that are several hours old.
It seems like there can be a queue that gets build up and then is suddenly flushed.
I see it happening the other way as well. This morning I made a post from my sh.itjust.works account and then switched over to a lemmy.world account and can't see it. So the delay is going both ways.
These posts are enlightening, I think reddthat is right lemmy will need to using batch messages on longer intervals soon, that's pretty unsustainable in current form. What an interesting scaling issue to encounter.