Weird performance increase in simple benchmark

using System;
using System.Diagnostics;
using System.Runtime.CompilerServices;

public static class CSharpTest
{
    private const int ITERATIONS = 1000 * 1000 * 1000;



    public static void Main()
    {
        PrerequisiteCheck();

        Console.WriteLine("running...");

        Test1();
        Test2();

        Console.WriteLine();

        Test1();
        Test2();

        Console.WriteLine("all done");
    }

    // I am having the issue with .NET 4.5.2 x86
    private static void PrerequisiteCheck()
    {
        if (Environment.Version.Major < 4)
            throw new InvalidOperationException("it seems you need a higher .net version");

        if (Environment.Is64BitProcess)
            throw new InvalidOperationException("try setting the platform to x86 instead");

#if DEBUG
        throw new InvalidOperationException("and btw this is not a release build");
#endif
    }

    private static void Test1()
    {
        Point a = new Point(1, 1), b = new Point(1, 1);

        var sw = Stopwatch.StartNew();
        for (int i = 0; i < ITERATIONS; i++)
            a = AddByVal(a, b);
        sw.Stop();

        Console.WriteLine("Test1: x={0} y={1}, Time elapsed: {2} ms", a.X, a.Y, sw.ElapsedMilliseconds);
    }

    private static void Test2()
    {
        var magical_speed_booster = Stopwatch.StartNew();

        {
            Point a = new Point(1, 1), b = new Point(1, 1);

            var sw = Stopwatch.StartNew();
            for (int i = 0; i < ITERATIONS; i++)
                a = AddByVal(a, b);
            sw.Stop();

            Console.WriteLine("Test2: x={0} y={1}, Time elapsed: {2} ms", a.X, a.Y, sw.ElapsedMilliseconds);
        }

        magical_speed_booster.Stop();
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private static Point AddByVal(Point a, Point b)
    {
        return new Point(a.X + b.Y, a.Y + b.X);
    }
}

public struct Point
{
    public Point(double x, double y) { X = x; Y = y; }
    public readonly double X;
    public readonly double Y;
}