/* Stop WP adding 'scaled' in the file name */
add_filter( 'big_image_size_threshold', '__return_false' );Technically, the above code will disable WordPress from scaling images. By default WordPress scales images to the media sizes registered in the ‘media settings’ and ‘theme settings’.
Example: When you add a full sized image with name hello.jpg, if the theme settings demand an image with 768×432, then the uploaded hello.jpg will be scaled to the said width and only the scaled image hello-768×432.jpg or hello-scaled.jpg will be served in the front end.
Regarding Performance and SEO perspective, you shouldn’t disable this function. If disabled, your mobile visitors are also served with full sized images which impacts your page speed as mobile visitors don’t expect images larger than 400px.