What djr33 says is generally true about the number of images and their byte size. These are like other images on the page, they have to load. The more total bytes they represent, the longer that takes and the longer it takes for the page to load.
But I'm familiar with this script. The problem you report usually happens when the total width of the images is greater than 9000 pixels. If that's the problem your images must be pretty wide - not a good thing if 11 or so of them fill up that space. But you can increase the total width allowed by increasing this number in the script:
Since it's negative, it's really decreasing. Anyways, make it like -90000 or even -900000.
document.write('<span id="temp" style="visibility:hidden;position:absolute;top:-100px;left:-9000px">'+leftrightslide+'</span>')
Try using smaller images though, and less images. The smoothness of the scrolling motion can also be affected if the images are large in byte size or even large in width and/or height.
The browser cache may need to be cleared and/or the page refreshed to see changes.
If you want more help, please include a link to the page on your site that contains the problematic code so we can check it out.