Do some performance optimizations

[darxtrix]

Loaded a directory of 346 images and each image ~15 MB of size. I waited for ~40 seconds but nothing loaded up. Consider doing lazy loading of previews, i.e load the previews of first few images and then accordingly with time load others as the user navigates. Loading all at the time, eats up a lot of RAM and slows down the app.

[sachinchoolur]

Hi,
Thank you pointing this out.
lightgallery loads only 3 images at a time. All the other images are loaded only when user navigate to different slides.
But currently it is not effective for thumbnails. I think the issue is because of that. I will add lazy-loading for thumbnails also