Control Asset's Export Quality
While exporting your InDesign document, you can choose to use the original image (which is the highest quality) or to let Object Export's settings as a guide.
Using Object Export you can decide the export quality per image asset. If you find the right settings, you can drastically reduce the download-time of your publication, while keeping the quality of your images.
The following settings are usually a good starter, but this may be different in your situation.