EPUB 3 large file leading to poor performance

I am generating very large epub3 books (almost all text) and I have noticed severe performance issues when trying to read them. Is there any format changes I can make to make reading performance better?

the files in my epub are as follows:

│   mimetype (1KB)
│
├───EPUB
│   │   cover.xhtml (1KB)
│   │   nav.xhtml (36KB)
│   │   package.opf (2KB)
│   │   s04.xhtml (5.3MB)
│   │
│   ├───css
│   │       epub.css (1KB)
│   │       nav.css (1KB)
│   │
│   └───images
│           cover.png (21KB)
│
└───META-INF
        container.xml (1KB)