StackOverflow - why does this code run faster when it's synchronized?

gistfile1.txt

package co.wds.visualvm;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.ReentrantLock;

//you will need to add commons-math-3.4.1 to the classpath
import org.apache.commons.math3.linear.MatrixUtils;
import org.apache.commons.math3.linear.RealMatrix;
import org.apache.commons.math3.random.GaussianRandomGenerator;
import org.apache.commons.math3.random.JDKRandomGenerator;
import org.apache.commons.math3.random.StableRandomGenerator;
import org.apache.commons.math3.random.UncorrelatedRandomVectorGenerator;

public class Main {
    private static ExecutorService exec = Executors.newFixedThreadPool(2);
    private static long startTime;

	private final static int MATRIX_SIZE = 500;
	private static UncorrelatedRandomVectorGenerator generator = new UncorrelatedRandomVectorGenerator(MATRIX_SIZE, new StableRandomGenerator(new JDKRandomGenerator(), 0.1d, 1.0d));
//	private static UncorrelatedRandomVectorGenerator generator = new UncorrelatedRandomVectorGenerator(MATRIX_SIZE, new GaussianRandomGenerator(new JDKRandomGenerator()));
	
	private static ReentrantLock lock = new ReentrantLock();
	
    public static void main(String[] args) throws Exception {
    	start();
    	
		for(int i=0; i < 100; i++) {
	        
			exec.execute(new Runnable() {
				@Override
				public void run() {
			    	double[][] matrixArrayA = new double[MATRIX_SIZE][MATRIX_SIZE];
			    	double[][] matrixArrayB = new double[MATRIX_SIZE][MATRIX_SIZE];
			        for(int j = 0; j< MATRIX_SIZE; j++) {
			        	matrixArrayA[j] = generator.nextVector();
						matrixArrayB[j] = generator.nextVector();
					}
			        
			        RealMatrix matrixA = MatrixUtils.createRealMatrix(matrixArrayA);
					RealMatrix matrixB = MatrixUtils.createRealMatrix(matrixArrayB);
			        
					lock.lock();
					matrixA.multiply(matrixB);
					lock.unlock();
				}
			});
		}
		
    	stop();
	}
	
	public static void start() {
		startTime = System.currentTimeMillis();
		System.out.println("Starting Tasks...");
	}
	
	public static void stop() {
    	try {
    		exec.shutdown();
			boolean finishedNicely = exec.awaitTermination(30, TimeUnit.SECONDS);  //blocks until tasks have finished, or 30 seconds have passed.  Change timeout if you run more tasks

			if(finishedNicely) {
				long total = System.currentTimeMillis() - startTime;
		    	System.out.printf("Processed tasks in %.3f seconds", ((double)(total/1000d)));
			} else {
				System.out.printf("Didn't finish nicely, timed out :(");
				System.exit(0);
			}
		} catch (InterruptedException e) {}
    }
}