No, but I'll do that right now and update this post when I'm done.

UPDATE: I just paged through 200 pages of the 1426 page The lord of the Rings in ePub format. I didn't read them of course, but I paused momentarily between page changes. The only slow downs I encountered were when changing to another chapter, which is expected since it has to open a new html file, and on rare occasions when a page of poetry was encountered. The latter only caused a slight decrease in page turn speed and affected only for the one page at a time. This too is probably explained by the way in which the publisher chose to display the poetry using html tags. I can take a peek later and see if the publisher used DIV tags.

But overall, I did not encounter the slow downs that at least one person has.