Limit req: "delay=" parameter. This parameter specifies an additional "soft" burst limit at which requests become delayed (but not yet rejected as it happens if "burst=" limit is exceeded). Defaults to 0, i.e., all excess requests are delayed. Originally inspired by Vladislav Shabanov (http://mailman.nginx.org/pipermail/nginx-devel/2016-April/008126.html). Further improved based on a patch by Peter Shchuchkin (http://mailman.nginx.org/pipermail/nginx-devel/2018-October/011522.html).