ELB's dependence on DNS CNAME record indirection is pretty crippling for web services that need to be very fast. In our case we need to have very good response time. In a quick performance test, the use of an ELB increased the average latency for HTTP requests by a factor of almost 2. This is mainly because the TTL on the CNAME lookup is zero. Thus, all lookups involve hitting name servers for two different domains, so the name resolution is way slower. (I worry that defeating caching in DNS is simply an abuse of the system.) The only hope for ELB in our case would be to get away from CNAME records by having Amazon support an Elastic IP as the address of a load balancer instance.
