Java’s linker is based on an open world assumption, where you can add arbitrary stuff to the classpath and dynamically link it at runtime. This defeats most compiler optimizations, so you’re left with the JIT.

GraalVM makes a closed world assumption, so it can do things like dead code elimination (for library functions that don’t get called), and apply static analysis to inline virtual method calls, perform constant propagation, and so on.

This lets it greatly outperform the JVM, at the expense of some rarely used functionality.

Basically, it’s like switching from the JIT to a C compiler’s -O3.