Java ConcurrentHashMap is better than HashMap performance wise?

I was just reading the book Clean Code and came across this statement:

When Java was young Doug Lea wrote the seminal book[8] Concurrent Programming in Java. Along with the book he developed several thread-safe collection, which later became part of the JDK in the java.util.concurrent package. The collections in that package are safe for multithreaded situations and they perform well. In fact, the ConcurrentHashMap implementation performs better than HashMap in nearly all situations. It also allows for simultaneous concurrent reads and writes, and it has methods supporting common composite operations that are otherwise not thread safe. If Java 5 is the deployment environment, start with ConcurrentHashMap

Note that in the above quote I used "[n]", where n is some number, to indicate the places where the author provided references, and as you can see he did not provide any reference for the bold part.

Not that I don't believe this statement, but I would love to know the supporting evidences of this statement. So, does anyone know any resources that shows the performance statistics for both ConcurrentHashMap and HashMap? Or can anyone explain to me why ConcurrentHashMap is faster than HashMap?

I probably will look into ConcurrentHashMap's implementation at work when I'm taking a break, but for now I would like to hear the answers from fellow SOers.