Thank you for all your information.
I think what you mean is that ImageMagick is some kind of light version of photoshop and convert images to sutiable size that fit for the ebook reader. Right?
So, if I want to get a same speed like the guy demonstrate in youtube,
I need to change all imgaes to 1872 × 1404 px (this is a Screen resolution of kobo auru one). Am I right?
