You don't need the Update in the Bullet Script

[RichP]

This is continually setting the Velocity every frame. You can just set it on Spawn and forget about it.

void Start()
    {
        PlayerMovement player = FindObjectOfType<PlayerMovement>();
        Rigidbody2D myRigidBody = GetComponent<Rigidbody2D>();
        float xSpeed = player.transform.localScale.x * bulletSpeed;
        myRigidBody.velocity = new Vector2(xSpeed, 0f);

    }