Many box shadows will make your app unusable on smartphones and tablets
Box shadows are awesome. Unfortunately they are also very costly to render. You will rarely notice the rendering time on a laptop or desktop PC, box shadows can make your app completely unusable on smartphones and tables. By "unusable" I mean "device freezes for 10 seconds after an user action".
But isn't it the future?
Not yet. Eventually mobile devices will become powerful enough to make this a non-issue. However, currently (August 2013) even high-end smartphones can become very unresponsive with a box-shadow heavy application.
Workaround
A quick workaround is to simply not render box shadows on mobile devices. To do so, use our touch device detection card to apply a touch_device class to the document <body>. Now you can add a rule to your CSS/Sass:
body.touch_device
*
-moz-box-shadow: none !important
-webkit-box-shadow: none !important
box-shadow: none !important