Divi already clears its cache automatically in all cases. It also tries to clear your WordPress plugin cache and your hosting cache, but Divi’s ability to do that is limited (although we do have almost every WordPress caching plugin covered). You shouldn’t ever need to turn settings on and off to reset something. I think people have seen that help because it’s essentially clearing their WordPress plugin cache when they click the save button in the Divi Theme Options. If they cleared their cache themselves, it would do the same thing.
Every time you do something that affects the entire website (install plugin, install theme, update plugin, update theme, save theme options, save theme customizer, save theme options, save divi presets, save global colors, save theme builder, etc), Divi will clear the cache for the entire site.
Whenever you do something that affects individual pages (save a page, save theme builder layout), it will clear the cache for those particular pages.
Regarding the new performance settings, it just comes down to what custom code you are working with in your child theme or plugins. Dynamic CSS and Dynamic Framework only load the CSS and PHP for the things that you use in Divi, so if you try to use those things outside of Divi, they aren’t always going to be present.
For example, are you using Divi module classes in your child theme, like taking the class from the Social Follow module button and adding it to a set of links in your Child Theme’s footer? That won’t work with Dynamic CSS, since the CSS for the Social Follow module will only exist when a Social Follow module is used. So that’s a situation were disabling Dynamic CSS will fix the problem by loading all of Divi’s CSS again on every page. That’s not a great trade off though, since now you are loading 800kb of CSS just to add a social icon in your footer. Much better to build your footer with the Divi Theme Builder and use the Social Follow module, that way you can keep Dynamic CSS enabled (or just use your own CSS in your Child Theme, instead of taking a class from one of Divi’s modules).