That will be triggered when your web requests are coming faster than your server can handle.
For example, if you have 4 unicorns and, on average, need 200ms to fully process a request, your capacity is 20 requests per second.
If, for a moment, you get 40 requests per second, the extra requests will sit in a queue waiting, and that message will be show. Also, you can’t go blindly setting up more unicorns, as the process to fully answer a web request involves a lot of moving parts that should be carefully evaluated.
Discourse gained a lot of those graceful degradation features recently as we work to scale out and handle some crazy level of traffic for the largest Discourse instances on the internet.