> It may sound obvious, but - optimizing your app to fulfill a request in 1/10 the time is like adding 9 servers to a cluster. Optimizing to 1/100 the time (reducing requests from say 1.5 sec to 15ms) is like adding 99 servers.

This cannot be correct. If you optimize 1/10, and then optimize to 1/10 again, that's the same as optimizing to 1/100. Does this "add" 18 servers or 99?

If you optimize to 1/10, then your cluster can handle 10x the load, which is the same effect you would get by simply having 10x the number of servers (assuming your app is perfectly parallelizable).

The article's quote is only correct if the "cluster" starts as just a single machine.