Hello Yoann, 

Apologies for replying a bit later, we don't work on weekends. 

Unfortunately, there is some conflict with the theme Divi, if you have it, the slow scrolling issue comes up. (If you don't have Divi, please try out the script on your pages and see if it helps for your theme as well.)

There are two solutions to this. Add this script: 

<script>
 var links = document.getElementsByTagName('a');
 for (var i = 0; i < links.length; i++) {
   if (links[i].href.includes('#')) {
     links[i].classList.add('et_smooth_scroll_disabled');
   }
 }
 document.querySelector('body').classList.remove('et_smooth_scroll');
</script>

to the pages where you have your Amelia shortcode. Or disable Smooth scrolling in Divi theme options.