Edit: After reading your comments, and testing some more, I must say that I've misunderstood how it all works.
I should've thought of Mastodon users like separate Lemmy communities...but not exactly. What confused me is the fact that you could look up a profile on a remote instance and see their posts, but they would be very delayed. On Lemmy, if your instance hasn't "discovered" a community, you wouldn't see it at all.
I followed a random user (whos posts were last synced many days ago), and it started syncing normally (it took ~1h for it to start, but it seems like it worked and now it's syncing their posts "in real time").
~~By accident I noticed that one instance had more japanese posts in the all feed than the other one. I thought maybe the other instance has certain languages filtered or they might be defederated from certain instances, but neither was the case. I found out that the other instance just fetches the posts from other instances much slower (days).~~
~~Then I decided to open 10+ (popular to fairly popular) instances and compare how quickly or slowly they sync with each other.~~
~~It's really bad and really random. Some instances sync perfectly with each other, some take hours, some take days, some take months...
I do not use Mastodon but if I did, finding that out would just make me not want to use it.~~
~~It reminds me of that time when there was a bug in Lemmy which made the federation broken, and that was very annoying, but we knew that there was a bug and that it was being worked on, and it was fixed fairly quickly.~~
~~But on Mastodon, from what I've seen, it doesn't even depend on the version the server is running, it truly just seems random.~~
~~It just seems odd to me that Mastodon (more popular and older software than Lemmy) would have such a glaring issue.~~
~~Wouldn't that be the first priority of every federated platform? For federation to work properly, because if it doesn't, then it can't compete with the centralized ones at all.~~
I'm not sure what it is you're comparing. Instances don't "sync" with each other. It's all based on the follow graph of the individual users of each instance. So yes, sometimes a post from one instance won't show up until days later on another because it just so happens that post may have been interacted with by some other user and only now it shows up on the instance.
FWIW, I operate multiple Mastodon accounts across multiple instances, and I've had no problem with seeing posts show up right away across instances.
To make this simple:
Lemmy: One user follows a community from another server, content federates from all users commenting and posting. It takes one follow to start that flow.
Mastadon: One user follows another, content federates for that one user. It takes a lot of follows to get significant content movement.
In addotion, it is much more likely that out of 10 servers, the same communities are followed vs the same people.