It’s the stock nginx error page. This can mean nginx is misconfigured. In this case, maybe it’s one of the servers in the cluster that’s down, but I’m guessing here because I don’t know how the site is set up.
Not so much slow, as just ignoring me. Click a link, nothing happens except progress bar moves a bit. Get bored with waiting, click link again and it works.
That’s a classic symptom of one server in a cluster being down. The load balancer hasn’t figured it out yet and sends a proportion of the requests to the dead server.