I feel like 3000px is still a bit on the large size, but regardless 1 full size image will take 1-2 seconds to load which isn't too bad.
The link you sent me is not a link to XenForo Media Gallery. It's an external link that contains images from XenForo Media Gallery. It has 11 images. All 11 images are the full size images (3000 x 2000 or 2000 x 3000) resized down with CSS to a much smaller size.
My point is, that page loads slowly because you're loading 11 full size images on it. We'd never be able to detect from the server side how big an image we need to display so the onus is on you to decide what size images to link to. Your choices are either the full size image, or the thumbnail size image.
If you want the page to load faster, the smaller images need to point to the thumbnail size image and only load the full size image once they've been clicked on.
Unfortunately due to the nature of what you're trying to do, it's way beyond the scope of support. You just need your pages to be serving the appropriately sized images (which are already available via the thumbnail size we already provide).