Wenson Hsieh of Apple’s WebKit team:
WebKit on iOS has a 350 millisecond delay before single taps activate links or buttons. WebKit has this delay because we also allow users to double tap to zoom, which is a great way to zoom in on content that is well-sized for large desktop displays, but appears too small on mobile devices. However, when a user has tapped once, WebKit cannot tell if the user intends on tapping again to trigger a double tap gesture. Since double tapping is defined as two taps within a short time interval (350ms), WebKit must wait for this time interval to pass before we’re sure that the user intended to tap only once. […]
On web pages optimized for mobile viewports, elements such as links and form controls are scaled to fit well on smaller screens. As such, double tapping on these elements increases the page scale by only a small amount. Since double tapping provides little value on these web pages, we’ve implemented a mechanism for removing the delay for single taps by disabling double tap gestures.