AWS Elastic Load Balancing: Seeing extremely long initial connection time

I think it is a possible ELB misconfiguration. I had the same problem when I put private subnets to ELB. Fixed it by changing private subnets to public. See https://docs.aws.amazon.com/ElasticLoadBalancing/latest/DeveloperGuide/elb-manage-subnets.html

Just to follow up on @Nikita Ogurtsov's excellent answer; I had the same problem except that it was just one of my subnets that happened to be private and the rest public.

Even if you think your subnets are public, I recommend you double check the route tables to ensure that they all have a Gateway.

You can use a single Route Table that has a Gateway for all your LB subnets if this make sense

VPC/Subnets/(select subnet)/Route Table/Edit

For me the issue was that I had an unused "Availability Zone" in my Classic Load Balancer. Once I removed the unhealthy and unused Availability Zone the consistent 20 or 21 second delay in "Initial Connection" dropped to under 50ms.

Note: You may need to give it time to update. I had my DNS TTL set to 60 seconds so I would see the fix within a minute of removing the unused Availability Zone.