1. Could you add this as option to theme setup? Just because this is impossible to setup in child theme and after update the changes will be lost.
I am not sure now, but we’ll focus on it
2. Where to change the code if I would like to apply to blog posts too? It seems that it doesn’t appear on blog category although the post template should be the same.
Lets have a look and see the difference on performance of first and second image
I am not clear this question, I checked this page, something wrong?