i create a page for a photoagency once. they had like 500 images per folder. my suggestions for you.
frontend:
- try caching list of images to json: Cache part of a page?
- store and display optimzied thumbs with something like the post api of imageoptim api and their php api
- create thumbs of images on hooks (panel.file.upload and panel.file.update) and not when page loads. or give kirby plugin imagekit a try.
- use somesort of lazyloading. in one project of mine i use jquery appear plugin since use isotope as well
backend:
- dont know much about panels caching but 1k images seems a but much for one page object to handle
- you could try to hack
panel/controllers/files.php >> thumb()function to display smaller or more compressed (or blured) thumb images. thus speeding up the backend thumbs generation.