The class that triggers the parallax effect is ".rellax" and can be used to any other elements where you want to add a parallax effect.

The code for js that changes the speed of parallax is found in app/js/src/script.js

 initParallax: function() {
   var $rellaxItems = $('.rellax');
   if( $rellaxItems.length > 0 ){
    // Also can pass in optional settings block
    var rellax = new Rellax('.rellax', {
    speed: 9,
    //center: true,
    round: true,
    vertical: true,
    });
  }
 },

The styles are found in app/style/scss/elements/_testimonialSlider.scss