Executioner.java

import javax.script.*;
import java.util.*;

class Expression {
  private Long id;
  private String name;
  private String expression;
  private String[] parameters;

  private Compilable engine;

  public Expression(Long id, String name, String[] parameters, String expression) {
    this.id = id;
    this.name = name;
    this.parameters = parameters;
    this.expression = expression;
  }

  public String getName() { return name; }

  public String invocation(String... args) {
    return String.format("%s(%s)",
        name,
        String.join(", ", args)
      );
  }

  public String declaration() {
    return String.format("function(%s) { return %s; }",
        String.join(", ", parameters),
        expression
      );
  }
}

public class Executioner {
  private static final int ITERATIONS = 10000;
  private final ScriptEngineManager manager = new ScriptEngineManager();
  private final ScriptEngine engine;

  public Expression[] database;

  public Executioner() {
    engine = manager.getEngineByName("nashorn");
    database = new Expression [] {
      new Expression(1L, "media", new String [] { "a", "b" }, "(a + b) / 2.0"),
      new Expression(2L, "resultado", new String [] { "x", "y" }, "media(x, y) + 20"),
    };
  }

  public Object run(Expression expression, String... args) throws ScriptException {
    return engine.eval(expression.invocation(args));
  }

  private void compileDatabase() throws ScriptException {
    for (Expression expression : database) {
      engine.put(
          expression.getName(),
          ((Compilable)engine).compile(expression.declaration()).eval()
        );
    }
  }

  public static void main(String[] args) throws ScriptException {
    Executioner executioner = new Executioner();

    double duration = System.nanoTime();
    executioner.compileDatabase();
    duration = (System.nanoTime() - duration) / 1000000;
    System.out.printf("Compilation\n");
    System.out.printf("%10.3fms\n", duration);
    System.out.printf("%10.3fms/i\n\n", duration / executioner.database.length);

    Expression expression = executioner.database[1];

    String[] a = new String[ITERATIONS];
    String[] b = new String[ITERATIONS];
    for (int i = 0; i < ITERATIONS; i++) {
      a[i] = String.valueOf(Math.random() * 5 + 3);
      b[i] = String.valueOf(Math.random() * 5 + 3);
    }
    duration = System.nanoTime();
    for (int i = 0; i < ITERATIONS; i++) {
      Object result = executioner.run(expression, a[i], b[i]);
    }
    duration = (System.nanoTime() - duration) / 1000000;
    System.out.printf("Execution\n");
    System.out.printf("%10.3fms\n", duration);
    System.out.printf("%10.3fms/i\n", duration / ITERATIONS);
  }
}

Na minha máquina:

Compilation
   191.580ms
    95.790ms/i

Execution
 14126.092ms
     1.413ms/I