The Kobo devices do multitask. If you start a WiFi sync, you can keep using the device while that is happening.
Personally, I don't think closing the book and returning to the home page would benefit from multitasking. The delay is probably not in getting rid of the book from memory, but in updating the status of the book, saving bookmarks and annotations and then retrieving the list of books to display. Because they all rely on the database, multitasking or threading might not help. Plus the last step relies on the first being done, so there are some dependencies.
My Glo takes 5 seconds to close a book, which does seem a long time. So, 25 seems excessive. How many books do you have on the device?