When you subscribe to a community, you only start receiving data from that point on. It doesn't go and download the history, that'd be very resource intensive on the remote instance. Thus it fetches a handful of posts, and no comments as a preview of the community, and after a couple days of activity is looks mostly normal.

Yes federation takes forever when you first start . .

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DO NOT WHITE LIST ANY INSTANCE

In case it helps, I only had Undetermined as my only language on my profile. Making sure that English was selected made everything show up. But it might have just been an accidental config on my end.

I see it