I've had a lot of issues trying to subscribe as well. Even searching in Jerboa has been a bit of an issue, I've had to go onto my computer and search for a community I know that exists and subscribe that way. It won't otherwise even show up in the app.

All things I'm sure will get ironed out soon enough. I quite like this whole thing, if I'm being honest. Doesnt have angsty reddit bullshit yet either, but I'm sure that will soon follow when the problematic mods start showing up.

Surely the servers are overloaded the refugees are always a stressing factor to infrastructure IRL, turns out that in the online life is kinda the same.

As far as I can tell, this is only an issue if you're the first one from your instance to subscribe to that community in the other instance. This is because your instance has to tell the other instance to start sending new posts its way. As well as any other subsequent updates. Lemmy instances work on a queue, so it may be a while before the other instance gets to that request if they have a lot of load.

EDIT: This doesn't seem to be just for the first person to subscribe. I guess so that the list/count of subscribers is accurate every subscription does indeed make a server-to-server transaction. The rest is accurate, though, your instance will have to tell the other instance to start sending updates over.