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.~~