I tested this on my 9500 book epub library. It took almost 6 or 7 hours to reach 100% but never gave me the screen to write the data. I even let it go for another 7 or 8 hours. The cause was the fact I included (without thinking) all of my ePubs, one of which was the 1913 edition of Webster's Dictionary. To it's credit it did not error and I'm guessing it would have finished someday. Obviously in hindsight I including a dictionary in the Flesch-Kincaid Grade Level calculation is just plain dumb.
On a side note the Flesch-Kincaid Grade Level number was interesting, but there are so many erroneous books listed from -0.4 to 4 (no doubt properly calculated) it makes it virtually useless to me. Still it is interesting and the test was a plus for me because I now have the ADE page counts listed to match my Sony eReaders.
Thanks for listening to folks and expanding the features of many of your plugins.