box-shadow transition performance

Adding a CSS transition to animate the box-shadow of an element is a handy trick.

It's a design technique that's often used on hover to highlight something. If you've used this effect you might have noticed that sometimes the performance can be sub optimal making the animation slow.

One trick you can try to improve the performance is moving the box-shadow to a pseudo-element.

Slower

Direct transition of the box-shadow property.

.box-shadow {  
  box-shadow: 0 3px 5px 0 rgba(0,0,0,0.08);
  transition: box-shadow 0.3s ease-in-out;
}
.box-shadow:hover {  
  box-shadow: 0 5px 15px 2px rgba(0, 0, 0, 0.1);
}
Faster

Pre render the hover state of the box-shadow with a pseudo-element and hide it by setting the opacity:0

.box-shadow::after {  
  box-shadow: 0 5px 15px 2px rgba(0, 0, 0, 0.1);
  opacity: 0; /* hide the hover shadow */
  transition: opacity 0.3s ease-in-out;
}

Apply the transition with the opacity property.

.box-shadow:hover::after  {  
  opacity: 1; /* show the hover shadow */
}