Ooops,

Also here, I found the answer by looking a bit closer.

The bottompart of the index.php shows this script part:

<script>

	jarallax(document.querySelectorAll('.jarallax'), {

		speed: 0.5,

		disableParallax: /iPad|iPhone|iPod|Android/,

		disableVideo: /iPad|iPhone|iPod|Android/

	});

</script>

The definition for jarallax is set here and used and at some module positions.
The setting of speed needs to be between 0 and 1.

(Hopefully this will also help others...)

Thank you for sharing your solution!

Ciaran