But how we can make it progressive?
What do you mean?
I mean to say Currently the Generated images[JPGs] using [File > Generate > Images assets] are not progressive.
Progressive images - " It's an image created using the JPEG suite of compression algorithms that will "fade in" in successive waves of lines until the entire image has completely arrived."
Till now, there's no way to make a progressive JPG file form Adobe Photoshop Generator.
If you what to see what else you can do with generator, please follow this links: