Performance gain for only including file on specific url

I’ve written custom code for my website in a single and rather large file.

This custom code for example generates the content of a few pages (using a shortcode).

When the code of a specific page is f.e. ~300 lines of code, would it be useful to check the url and only include that part of code when on a specific url?

So basically, does having functions that you don’t use influence your server’s performance or is it something not to worry about?