There is a ~300ms delay on mobile browsers while the browser waits to see if it's a single or double tap (since double-tap is for zoom).
This can be removed on recent builds of Android devices by using:
<meta name="viewport" content="width=device-width">
However, Mobile Safari doesn't follow this same logic, and there's no way to disable this delay.
You can read more about this here: http://updates.html5rocks.com/2013/12/300ms-tap-delay-gone-away