in assets/js/slick.js [933:967]
    Slick.prototype.fadeSlide = function(slideIndex, callback) {
        var _ = this;
        if (_.cssTransitions === false) {
            _.$slides.eq(slideIndex).css({
                zIndex: _.options.zIndex
            });
            _.$slides.eq(slideIndex).animate({
                opacity: 1
            }, _.options.speed, _.options.easing, callback);
        } else {
            _.applyTransition(slideIndex);
            _.$slides.eq(slideIndex).css({
                opacity: 1,
                zIndex: _.options.zIndex
            });
            if (callback) {
                setTimeout(function() {
                    _.disableTransition(slideIndex);
                    callback.call();
                }, _.options.speed);
            }
        }
    };