Image size for post slider - reduze file size or have seperate files?

I’m trying to understand how to best choose image sizes. Atm I have some very large image sizes, because I want clients to be able to download high resolution images. In posts and highlighted images etc it seems to load fairly fast - I assume wp uses some automatically scaled versions. But the slider loads slow. Is the best way to upload 2 seperate image files, one reasonably sized for the webpage - and a seperate high resolution image for download? Or is there some other neat way to solve this?
https://217704-demo.web.tornado-node.net

Hello,

Well, I think that two separate uploads would be the easiest way.

Please feel free to ask any other questions that you might have.

Kind Regards, Roman.