Does creating multiple tpl files affect site performance?
It won't make a noticeable difference as long as you disable rebuilding the theme registry on production, enable caching, and follow best practices (e.g. don't execute logic or queries in your templates, and never ever save PHP code in the database). There are quite a few other, more noticeable things you can tweak to improve performance and load time before you get to the level of worrying about the number of templates.