unity3d – Object Generation Gaps

Help me understand what the problem is in Unity3D.
I generate an infinite number of blocks in motion.
But the higher the speed of movement, the larger the gap between objects. As far as I understand, the gap will be sampled due to the frame refresh time.

How to generate objects close to each other?

void Update()
{
    if (startspawn)
    {
        spawn1 = Instantiate(spawner);
        spawn1.GetComponent<Rigidbody>().velocity = new Vector3(15, 0, 0);
        startspawn = false;
    }
    if (spawn1.transform.position.x - startcoordinateX > size)
    {
        spawn2 = Instantiate(spawner);
        spawn2.GetComponent<Rigidbody>().velocity = new Vector3(15, 0, 0);
        spawn1 = spawn2;
    }
}

Screenshot