As per the comment here on the Microsoft Phone Dev Center forums from Rob Caplan (a Microsoft employee),
"This is not adjustable by apps. It is a user setting. I believe that on Windows systems it will be based on the double click rate (most UI timings are based on that). I'm not sure on the phone. There may be an adjustment in the ease of access centre."
There is no such adjustment on Windows Phone 8.1 Developer Preview in the Ease of Access settings.
It seems like there is nothing I can do here. Perhaps it is possible to code your own gestures in Silverlight?