Your site sits on a server (computer).
The more complicated your site is, the more memory it needs to do things and process tasks. WordPress has a built-in function to request a certain amount of memory allocation from your server to operate the site.
One thing that always slows down WordPress sites is when the site needs more memory than it’s been allocated.
This is often a sign that something isn’t right on your install.
Also, remember that the cheaper your hosting package, the less memory usage WordPress is going to have.