One way is put the URL of the community you want to follow in the search box; that's how I'm able to follow /c/lemmy from lemmy.ml on the mastedon server I'm using.
Since you're on a lemmy server, you can also switch between Subscribed, Local, and All at the top of the main feed. "All" is all communities from all federated instances that _someone_ on your home instance already subscribes to. If you see something in the All feed you like, you can join that community from there.
@nutomic @Lobstronomosity In one of the comments I thought I saw that the biggest CPU load was due to image resizing.
I think it might be easier to split the image resizer off to its own worker that can run independently on one (or more) external instances. Example: client uses API to get a temporary access token for upload, client uploads to one of many image resizers instead of the main API, image resizer sends output back the main API.
Then your main instance never sees the original image
I did what I said... I put the "URL of the community" (https://lemmy.ml/c/lemmy) into the search on my mastodon server.
It sounds like you found a shorthand.