How do I change the scrolling speed?
In the file back_to_top/js/back_to_top.js you'll find the following on line 23:

$('html,body').animate({ scrollTop: 0 }, 1200, 'easeOutQuart', function() {


The 1200 is the animation duration in milliseconds. Lower it and the animation will go faster. Optionally you could raise a feature request in the module issue queue to make this a configurable setting.