I installed the child theme as you said. Then I added the code you specified to the function.php file. But the problem still persists. There was very little acceleration. Also, after adding the child theme, my site speed dropped.
As we can see, your add-to-cart time now is ~300-700ms which is quite fast. And in general, the child theme doesn’t affect your website performance anyhow.
Add to cart as you said yes. But after installing the child theme, my site speed dropped from 98% to 57%. Also, there was a problem in the tag manager. I can’t access the live chat I added.
It is not related to the child theme since it doesn’t add any styles or scripts to your website. You can switch to the parent theme and see that the speed will be the same.