Many reasons possible but what you can control are the file size of the "large Image" - how large are these?Customers are reporting a 3-4 second delay between clicking the image and when the large image actually pops up.
You can optimize these and make them a uniform physical size like 500x400px and then reduce the size of the file to match as a monitor only displays web content at 72dpi.
Kim's faster to post!!



