Yes.

The "legacy" version could stream the image's binary data a little at a time, since it was able to use the XPCOM interface to access Firefox's functionality.

The WebExtensions version is much more restricted, because it is treated much the same as any web page, and so it has to download the entire image before it can process it.

Out of curiosity, what are your browser's cache settings? I'd hope that the Viewer's request for the image has access to the cache, but maybe not.

- Alan