Placeholders

The purpose of Placeholder text (or Image) is primarily in creating a template with areas of replaceable text that you can click to overwrite with your own content. Many of Apple's templates incorporate placeholder text, notable by its Latin (Lorem Ipsum) content.

Placeholder images are those images that you size to the exact placement and dimensions that you want in a template, and during document development, you can drag and drop, or replace the placeholder image with another. The new image will automatically rescale to the dimensions of the placeholder image settings.

This issue is not placeholder text or image, but the original image size residing within the Pages document. You can choose File menu : Reduce File Size… and this will reduce the dimensions and size of the respective images in the document, without reducing their original resolution (dpi). Or, by understanding the target for this Pages content, can you get by with lower dpi instead of 300 dpi for reduced image size prior to using the File menu reduction command?

For example, I dropped a 1024x677 300 dpi JPEG image of size 441KB into Pages v7.3. After the Reduce File Size… operation, that image was 938 x 620 and 300 dpi of size 114KB. Because the visual selected area of the image in Pages was smaller than the new document dimensioins, my image layout was unaffected. But adding images to Pages creates additional internal document baggage, and its size will balloon anyway.

Pages v5 thru v7.3 releases are notoriously intolerant of large documents — especially if inundated with large dpi images. Pages will crawl with these huge documents. This is not something Apple has addressed in five years, and I don't believe they are motivated to address this issue.