I managed to write a code using Jquery. It's not to most elegant way, basically its made to click the next arrow in the slideshow every couple of seconds:
<script src="//ajax.googleapis.com/ajax/libs/jquery/2.0.2/jquery.min.js"></script>
<script>
const init = () => {
setInterval(() => {
$('.gallery-reel-control [aria-label="Next Slide"]').trigger('click');
}, 6000); // change this value to change the speed
}
document.addEventListener('DOMContentLoaded', init);
</script>
You can insert this into Code Injection or in to the Advanced section of the specific page the Slideshow is on.
I haven't tested if this works on any other gallery types by the way.