Wouldn't it still be a bit less taxing even with only one user? If I'm not mistaken then your instance only initially requests a community/post/comment from another instance when you specifically search for it. After that your server gets updates through activities pushed by the other instance. So if you refresh a post multiple times those requests only go to your instance. It somewhat acts like a cache, while the other instance can push activities at it's own pace instead of being hammered with requests. Of course multiple users per instance would still be better.