We recently performed a load test on our servers and it resulted in pretty decent numbers. Average response time ~75 ms.
Since we only have around 100 visitors a day (we didn't market the product yet since we are in beta and keeping it public but not actively promoting the product) we use small machines to reduce the costs (currently the servers cost us ~$50).
We were curious about how our system would hold on heavy load as it turns our the system holds very well and should hold around 400000 visitors a day.
P.S. We are planning to further optimize and will be using more powerful machines next year.