As noted in MONGOID-4715, the default timeout for nginx is 30 seconds, which also happens to be our default for server selection. This has the potential of preventing the user from seeing the true cause of request timeout.

The server selection spec indicates that the "default value was chosen to be sufficient for a typical server primary election to complete. As the server improves the speed of elections, this number may be revised downward," maybe it's time to do that.