Use calibre to edit the eBook. Given that it came from Word, chances are there is a lot of garbage in the ePub that you can remove. Also check the size of the images. If they are beyond 1500 lines, reduce them in size to 1500 lines. Calibre can do this in the editor. If there are any embedded fonts, remove them. Make this as streamlined as possible. Also, check the code using epubcheck. There is a Calibre editor plugin to allow you to use
epubcheck to check the eBook. Also, check the CSS and remove any excess lines from the classes. And if you see things like <p class="someclass">, replace it with <p> and and define <p> as what <p> is going to be most of the time.