Sorry, it is the linked image size that is taking time to load.
It is unfortunate that WordPress native media uploader, until now, has only convenient link to original file (no selection of thumbnail, e.g. large) unless we are willing to modify the linked url individually (or a plugin, which I haven't identify one that work in my setting). So, for those images that I didn't reduce before upload, they will pop up slow.
I wonder if your plugin can supplement this missing functionality in WordPress native media uploader. With plenty of bandwidth here while realizing viewers have limited bandwidth, my ideal workflow for handling image is: (1) upload full resolution image (for keeping and future thumbnail regeneration); (2) insert medium size image in post (for fast browsing experience); (3) link to large size image (for larger view). WordPress media uploader allows me to do (1) and (2) but not (3). WordPress mobile app handle images rather thoughtlessly by fixing all three into the same selected size defeating the purpose of generating multiple thumbnail sizes.
A few thoughts from user point of view (obviously, easier said than done when thinking as developer):
1. An option to allow the lightbox to load selected size of thumbnail (e.g. large) rather than the linked file (which may be at full resolution).
2. At the moment, the display goes blank with loading image (rolling animation) when the slideshow loads the next image. This gives sluggish feel (although it's the image size that causes the problem). I wonder if there is way to load the next image while current image is being displayed (hence the timing setting will be adjusted accordingly).
3. To further reduce unnecessary delay and data usage to load large image to be displayed on small screen (e.g. mobile devices), I wonder if it will be possible to dynamically select and load appropriate thumbnail size in accordance to the intended display size. In this way, users can choose to capitalize this ability to speed up their website image loading by generating different sizes of thumbnails.
4. Another user experience point. The swipe works but it didn't feel like a swipe. Perhaps an animation of the image sliding left/right will enhance that feeling.
Sorry for the long comments. This is a great plugin. Thanks again!