Why incrementing transform.position causes jitter on iOS

Hello,
I am having some problems with a simple script which causes jitter on the sprite.
All the script does is
public float speed; void Update () { var pos = transform.position;pos.y += speed * Time.deltaTime;transform.position = pos; }
This code runs fine in Editor, or standalone builds. But when I try to run it on my iOS device, you can see some jittering. The scene has 3 gameobjects with the above script and there is no rigidbody physics. Its just three objects falling without any physics.
Edit: The FPS on iPhone is a constant 30 which makes it more weird

Any idea why it causes jittering?

Thanks

have you tried using int rather than float? Just wondering if maybe it has something to do with the numbers after the floats decimal point? Of course this will make your game run strange but it might help you narrow down the issue?