Add Performance vs scalability section

README.md

@@ -531,3 +531,17 @@ Next, we'll look at high-level trade-offs:
 Keep in mind that **everything is a trade-off**.
 
 Then we'll dive into more specific topics such as DNS, CDNs, and load balancers.
+
+## Performance vs scalability
+
+A service is **scalable** if it results in increased **performance** in a manner proportional to resources added. Generally, increasing performance means serving more units of work, but it can also be to handle larger units of work, such as when datasets grow.<sup><a href=http://www.allthingsdistributed.com/2006/03/a_word_on_scalability.html>1</a></sup>
+
+Another way to look at performance vs scalability:
+
+* If you have a **performance** problem, your system is slow for a single user.
+* If you have a **scalability** problem, your system is fast for a single user but slow under heavy load.
+
+### Source(s) and further reading
+
+* [A word on scalability](http://www.allthingsdistributed.com/2006/03/a_word_on_scalability.html)
+* [Scalability, availability, stability, patterns](http://www.slideshare.net/jboner/scalability-availability-stability-patterns/)