Hi Karen,
Thanks for reaching out to us.
First I would like to say that unless you are dealing more than 10.000 Categories/Tags I wouldn’t worry about this kind of optimization, there other ways to speed up the website by using caching, I’m talking advanced level caching not only plugin based.
So as you asked I will point you to first the Codex with the WordPress Schema and there you will be able to see a Table called <b.wp_terms, the idea is that any category is a Term and the group called “Categories” is a Taxonomy.
WordPress uses 3 tables manage that information:

I would only go to that place of micro optimization if you really know what you doing with PHP and WordPress development.
My Best Regards,