Splitting admin code from the main plugin file to optimize site performance
As mentioned in Chapter 2, Plugin Framework Basics, the entire content of the main code file of a WordPress plugin gets evaluated every time any page is rendered on the site, whether it's a visitor-facing page or a backend administration page. This means that large amounts of PHP code can potentially be parsed on every iteration, wasting processing cycles on the site's server, even though some of this code will never be active when regular visitors are browsing the site.
A prime example of this waste is all of the code samples that we have been building in this chapter. While this code is extremely useful for site administrators, there is no sense in having the web server parse and validate that code when regular pages are displayed. For this reason, it is better to isolate this code in a separate file that will only be loaded and parsed when someone is visiting the site's dashboard.