All of this. NodeJS is a valid tool, yes, but not the sole reason for those performance improvements.
Take the PayPal case, which (iirc) is mostly based on a 10 year old Java stack. If they were to create a new project from scratch, using current-day Java technologies and frameworks based on current-day paradigms (technologies like Play, Akka, Vert.x), they would get equal if not even higher performance improvements.
I don't know how developer productivity and happiness would be using those tools, but the Java development tools are very mature (compared to JS), and happiness... I don't know really, I'm inclined to believe a lot of developers use NodeJS because it's new and hot and Java isn't, instead of looking at their customers' needs.
Take the PayPal case, which (iirc) is mostly based on a 10 year old Java stack. If they were to create a new project from scratch, using current-day Java technologies and frameworks based on current-day paradigms (technologies like Play, Akka, Vert.x), they would get equal if not even higher performance improvements.
I don't know how developer productivity and happiness would be using those tools, but the Java development tools are very mature (compared to JS), and happiness... I don't know really, I'm inclined to believe a lot of developers use NodeJS because it's new and hot and Java isn't, instead of looking at their customers' needs.