I notice that your images are of quite a low resolution. (The first image in your gallery is only 600px x 420px.)
Try increasing the resolution of your gallery images. The default resolution for gallery images when using JuiceboxBuilder-Pro to create a gallery is 1024px x 768px (over 3 times as many pixels as your current first image).
Don't worry about your images being too large for your gallery. Your gallery uses the default value for the imageScaleMode configuration option (SCALE) so Juicebox-Pro will dynamically scale down large images so that they fit within the gallery's image area (whilst respecting their aspect ratios and without cropping).
When allowing JuiceboxBuilder-Pro to resize images for a gallery at the default resolution (1024px x 768px) and quality (80%), the resulting images should have a filesize of around 120KB so the images should take no longer to load than your current images.
Just increase your image resolution slightly and you might also like to try bumping up the quality a little (try 90%).
This should help.