[enuddleyarbl]

In that last case, it was Ben Aaronovitch's "Lies Sleeping" (Rivers of London #7). But, again, there's nothing odd in that book (or any of the others where it's happened) that I can find. All it's CSS has been replaced with a subset of my standard set, I run the Calibre Editor's "Check Books" and the Epubcheck plugin on it without problem, and the delay happens only once. Since I can't find any common denominator for the times this happens, I tend to agree with Kovid and think it's some issue with Windows. But, what it is or why it's happening, I don't know.

EDIT: I also run the editor's "Remove Unused CSS Rules," "Fix HTML," and "Beautify All Files" on the book.