This is because Avenza Maps is raster based software (like Adobe Photoshop) and not a vector based (like Adobe Illustrator). All vector data is rasterized on import. It’s not only the resolution but also the image size that will affect the final look of the map, the amount of zoom you can achieve and the files size. The file size will then affect processing times on import. We don’t normally recommend image sizes above 10,000 x 10,000 but this is possible as an image may have a very high resolution but a small disk size due to image compression. I would recommend trying some different export settings and image dimensions until you find something that suits your needs.
For more help and guidance, you can contact us via help.avenza.com.