With regard to the speed at which your gallery images load, there are several contributing factors.
(1) The speed of the web server hosting the site.
(2) The speed of your own internet connection.
(3) The file size of the gallery images.
(4) The value of the imagePreloading configuration option.
You have little or no control over #1 and #2 is not relevant to Google search results (but can certainly play a part when viewing a gallery yourself).
With regard to #3, make sure that your image file sizes are not too large.
If using JuiceboxBuilder-Pro to create your gallery with default image dimensions (1024px x 768px at 80% quality), then resulting gallery images should be somewhere within the range of between 100KB and 200KB each.
If your image file sizes are much larger than this, then try tweaking the image dimensions and/or quality to bring the file sizes down to a reasonable level for web gallery use.
With regard to #4, the imagePreloading configuration option "Determines how images are preloaded. Also controls if images are kept in memory after being viewed." (Please see the Main Image Options in the Config Options page for details.)
The default value for imagePreloading is PAGE (whereby all images on the current thumbnail page are preloaded) but you could change this to NEXT (so that, when the gallery is initially loaded, only the next image in the gallery is downloaded and cached by the browser) or ALL (so that all gallery images are initially downloaded) to see if this makes a difference.
imagePreloading is supported by Large Screen Mode only so I don't think it'll have any effect on Google's mobile search results (Juicebox uses Small Screen Mode on mobile devices unless screenMode is explicitly set to LARGE in JuiceboxBuilder-Pro's 'Customize -> General section) but you might like to try changing its value as this is the only configuration option which has anything to do with the loading of the images.
Unfortunately, I know of no other ways to influence Google search results as far as image loading is concerned.