Hello,

1) No, using the child theme could not make the page or website slow.

2) When an update comes the parent theme is updated.

3) You don’t need to translate the child theme again if you translated the parent theme. You need to translate only the parent theme.

4) Only the code customizations you made will not work that you made to the child theme when you activate the parent theme, not the CSS you added.

You can read the following article for further help:
https://www.wpbeginner.com/beginners-guide/wordpress-child-theme-pros-cons/

Best Regards.