The Graal compiler is compiling Java code to faster machine code on top of the JVM. Also GraalVM's JavaScript implementation uses an entirely different approach by using Truffle for its implementation making it significantly faster. Also GraalVM ships with Node.js support, tooling and other languages.

Here is a full list of reasons: http://www.graalvm.org/docs/why-graal/

Here are some (maybe a bit outdated, but independent) benchmarks comparing it to Nashorn: http://blog.nidi.guru/tech/javascript/2018/04/02/javascript-...

We have some way to go to beat V8.