Slow response time for `$.mobile.loading()` on mobile devices

Slow response time for `$.mobile.loading()` on mobile devices

On my site, I have a button B which onClick triggers the following code:
$.mobile.loading( 'show', {});
bigJSONrequest(); // synchronous request (NO AJAX)
setTimeout(function()
$('.landing').slideUp('fast', function() {
$.mobile.loading( 'hide', {});
}); }, 1500);
The page-loading widget (triggered by shows up $.mobile.loading)
immediately on desktop computers.

But when I try this on a mobile device (even with fast WiFi connection),
there is a rather large delay until the $.mobile.loading widget appears.
In fact, when I click button B, the network spinner on my iPhone triggers
immediately. Shouldn't the call to $.mobile.loading be entirely
client-side?
How can I make the $.mobile.loading show up faster on mobile devices?
Example: I wanted to make a JSFiddle but it proved to cumbersome. Feel
free to visit here, from your desktop and mobile device and see for
yourself.