I think it’s still considered good practice to compress those images before uploading (although WordPress does do some of the compression work for you too).

I would go ahead and upload just the reduced 1200×800 versions (or whatever ratio you decide to go with)

The resize behavior is by default limited to crop or don’t crop, but you can add custom sizing logic through filter hooks. For example, I added a size that downsizes to the specified size using the longest image dimension, whether it be landscape or portrait (usually it resizes to width regardless of orientation).

The other advantage of doing this is the different sizes then are included in the image srcset and sizes attributes so mobile browsers can fetch the most efficient size for their screen. Because of limited bandwidth of some mobile connections, image size is just as important as it ever was.