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...)
