We have 25 instances with one load balance on Ec2. If we hit any single one the speed is fast(<300ms) but if we us AB to test multiple connections we get poor results often with an average of 1800ms. Does anyone know if this is EC2 limiting us, we're only hitting these servers from 27 outside servers with about 2 million requests and hour so maybe they think it's a DOS?
We've loaded tested applications with up to 2.8 million requests with an average round trip latency of under 100ms. Based on this, and the fact that you're using slightly fewer requests, I doubt you're being throttled.
Having said that, this question was asked in 2009 (over 2 years ago at the time of writing), so it's entirely possible that AWS was doing things differently back then.