The metainfo dashboard is a system based on single URL with the main purpouse to make it easier to manage meta link by link instead that going through the complex Joomla override system, component, category, menu, article, etc

Obviously the system addresses unique links, so if you change l inks you change/lose metainfo associated. You can fill them in the live site or fill them in the staging domain then export/import as can be done when you change domain and explained at this FAQ: https://storejextensions.org/faq/i-changed-the-website-domain-how-may-i-avoid-to-lose-all-metainfo.html

The question of performance/override makes no sense, there is no impact on the performance of the Joomla execution when setting a certain value for the metatitle, metadesc, etc

The metainfo dashboard would not be done to be completely filled in as when you click auto populate, current meta values can be seen in the SEO Spider. However someone like to see everything filled in with the current Joomla values, so there is the possibility to use this feature: https://storejextensions.org/faq/opening-the-search-engines-metainfo-dashboard-it-s-empty-why.html

Best,
John