I'm running into a problem where long running requests such as downloading a CSV from a slow database or a large database eventually causes a timeout for a reverse proxy. I can increase the timeout period on the proxy to work around this is most cases.
Another solution to this issue is suggested by AWS:
https://docs.aws.amazon.com/elasticloadbalancing/latest/application/application-load-balancers.html#connection-idle-timeout
They suggest to "send at least 1 byte of data before each idle timeout period elapses". I think if Metabase can do this for CSV exports it would be better than just increasing the timeout to an arbitrary value..