Having a child theme does not itself create performance issues, since child themes load only their files and the files of the parent theme that their files do not replace.
In short, if you did replace the parent theme with the child theme, there would be no performance difference whatsoever.
The point of a child theme is so that the parent theme can be updates safely without replace customization within the child theme, so replacing the parent theme with the child theme would also defeat the purpose.
In short, without a child theme, as soon as a theme update is released, you’d lose all of your customizations.
Instead, we have some general recommendations for site optimization at https://projectdmc.org/documentation/article/optimization/
Keep in mind that the only way to get a perfect score in speed tests is to have a plain text static HTML site with no images or styling, so find your own balance between functionality and speed.
In the end, copying the child theme and pasting into the main theme was very quick. Two hours. It was enough to copy from the staging site to the live site.
If I had thought about it before, the staging site created with a plugin created problems.