c# – 线程缓慢且难以预测？

public void Trace(int start,int jump,Sphere testSphere)
{
    for (int x = start; x < scene.SceneWidth; x += jump)
    {
        for (int y = 0; y < scene.SceneHeight; y++)
        {
            Ray fired = Ray.FireThroughPixel(scene,x,y);

            if (testSphere.Intersects(fired))
                sceneRenderer.SetPixel(x,y,Color.Red);
            else sceneRenderer.SetPixel(x,Color.Black);
        }
    }
}

SetPixel只是在一维颜色数组中设置一个值.如果我通过直接调用它来正常调用函数它以恒定的55fps运行.如果我做：

Thread t1 = new Thread(() => Trace(0,1,testSphere));
t1.Start();
t1.Join();

它以50fps的速度运行,这很好并且可以理解,但是当我这样做时：

Thread t1 = new Thread(() => Trace(0,2,testSphere));
Thread t2 = new Thread(() => Trace(1,testSphere));

t1.Start();
t2.Start();

t1.Join();
t2.Join();

它遍布整个地方,在30-40 fps之间快速移动,有时超出50或者降到20,它根本不是恒定的.为什么它比在单个线程上运行整个事情时运行速度慢？我正在运行四核i5 2500k.