To solve an issue with ACF I need to add some lines of php to GP’s functions.php that will not be overwritten by future updates. What is the most efficient way to do this? Through adding a Child Theme of using Code Snippets? I should mention that I have no experience with Child Themes, and also the speed of the website is key.
yes they should go in a Child Theme function file or the easiest option is to use the code snippets plugin. Neither option will be detrimental to your sites performance.