Нашёл такое решение:
<div class="flexslider">
<ul class="slides">
<li data-duration="100">
<img src="https://upload.wikimedia.org/wikipedia/commons/c/ce/Burger_King_Restaurant_in_Sherman_Oaks%2C_California_-_panoramio.jpg" />
</li>
<li data-duration="500">
<img src="https://upload.wikimedia.org/wikipedia/commons/a/ae/Canoga_Park%2C_California_-_panoramio.jpg" />
</li>
<li data-duration="5000">
<img src="https://upload.wikimedia.org/wikipedia/commons/c/cd/CHS_Building.JPG" />
</li>
<li data-duration="1000">
<img src="https://upload.wikimedia.org/wikipedia/commons/e/e4/First_Street_Bridge._Downtown_Los_Angeles%2C_CA._Looking_West._-_panoramio.jpg" />
</li>
</ul>
</div>
$(document).ready(function() {
var slider = $('.flexslider').flexslider({
slideshowSpeed: 6000,
directionNav: true,
slideshow: 1,
animation: 'fade',
animationLoop: true,
after: function(slider) {
slider.stop();
slider.vars.slideshowSpeed = $(slider.slides[slider.currentSlide]).data('duration');
slider.play();
}
});
});
https://jsfiddle.net/crazzzy/xs67kv18