The issue: currently all external images load as full-sized images, even if they’re only displayed as thumbnails on the site. This is extremely detrimental to page sizes and page loading times, and goes against what the External Images plugin is trying to achieve. I do not need 2,000 pixel images to load in 300×300 pixel spaces, for example in the product image galleries.
Solution: Use Jetpack’s free Photon CDN via their API to automatically display images at thumbnail sizes when required. Use the Photon API (https://developer.wordpress.com/docs/photon/api/) to check if it’s installed and enabled, and if it is, rewrite the URLs to run through Photon so they will return an optimized thumbnail sized image instead where needed. You could make this an option in the admin panel, and greatly improve loading times on the Product Page and all Archive pages a product may appear on.