[CoffeeDregs]

Another gotcha is that ELB appears to load balance based on the IP addresses of the requests... We had private VPC/IP workers talking hundreds of requests per second to a non-sticky-session, public ELB fronted service (... don't ask why ...) and experienced really strange performance problems. Latency. Errors. What? Deployed a second private ELB fronting the same service and pointed the workers at it. No more latency. No more errors.

The issue appeared to have been that the private IP workers all would transit the NAT box to get to the public service and the ELB seemed to act strangely when 99.99% of the traffic was coming from one IP address. The private ELB saw requests from each of the individual IP addresses of the workers and acted a lot better. Or something.