iBooks slowly reading compiled epub

I’ve compiled an epub file from my novel (~420000 chars, the file is ~200 Kb) and I’ve found that iBooks is very very slow when reading the book, both on iPhone and iPad. It takes a lot of time to change page (I swipe the finger on the screen and nothing happens for one or two seconds). This happens only with the compiled epub; all my commercial ebooks work fine.
What can I do?