I just installed ZP and so far I really like it. I uploaded some images from my google Photos which are hi-res photos.

Is there a way to scale back the images for loading without uploading a lower quality image? I want to be able to allow people to download the original file if possible but increase load times.

Down-sizing images will always affect image quality. (throwing pixels out)
Up-sizing a low quality image will never improve the quality. (making up new pixels)

Zenphoto works somewhat different.
You can upload your full sized images.
It is the theme (and the functions and CSS used) that determine how big an image is displayed on the website.
Your original full image is not changed in any way.
If the theme uses the right functions/plugins you can let visitors download the original full image.

The only limitation is your server.
Processing large images requires a lot of computer resources.
Plenty examples on the forum where thumbnails/sized images were not generated. Especially on shared hosts.

Useful user guide article related to that: https://www.zenphoto.org/news/caching/

Thanks. I see that once I have viewed every image, they load much faster the second and future times. That is good then.