[smoores]

Yup, great point. Especially if you've used some tool to convert from another file, like a PDF, into an EPUB, you can easily end up with the entire book in a single XHTML file, which, again, can be pretty heavy for a browser to parse and format! I also have no idea whether Calibre et al actually use native web views, or have their own renderers, which are almost certainly less performant than native web views!