Classic Load Balancers should have cross-zone load balancing enabled

How to fix it

Turn on cross-zone load balancing on the Classic Load Balancer, so each of its nodes can reach registered instances in every enabled zone rather than only its own. Without it, traffic is split evenly between zones before it is split between instances, so uneven instance counts mean uneven load. Cross-zone traffic is not charged on a Classic Load Balancer. Having only one zone enabled is ELB.10 instead.

In the console

EC2 -> Load balancers -> the load balancer -> Attributes -> Cross-zone load balancing

With the AWS CLI

aws elb modify-load-balancer-attributes --load-balancer-name <load-balancer-name> --load-balancer-attributes CrossZoneLoadBalancing={Enabled=true} --region <region>

With Terraform

resource "aws_elb" "this" {
  cross_zone_load_balancing = true

  listener {
    lb_port           = 80
    lb_protocol       = "http"
    instance_port     = 80
    instance_protocol = "http"
  }
}