I tried it on the sample data posted above by BrowserUK, with the single answer LCS word string returned in very little time (consistently less than 0.02 sec

Cool. Now think about doing all the 3,206,778 pairing of 2533 sentences. 3,206,778 * 0.02 = 64135.56 seconds = 17 hrs 49 minutes. That's why I was somewhat impressed with my "few seconds under 10 minutes".

If you would like the 2533 sentences I extracted from Huckleberry Finn in order to perform a real comparison, /msg me an email id and I'll forward it to you.