The Rellax extension allows you to apply a parallax effect to any widget, following the page scrolling.
You can use the Rellax animation on your pages, where you want to have a different scroll speed for your element from the other elements.
Speed: to set the animation speed
Percentage: you can take it as a point of height where the original position will meet. 0.5 will make it meet it at the center.
z-index: CSS property to make the element stack on the top of other elements.