How fast has the English language changed?

In the Icelandic sagas, the Norse speaking Egil Skallagrimsson and his brother hired themselves out as mercenaries to one of the Anglo-Saxon kings in what is now England, and with some effort he could understand and be understood by the locals even though they spoke different languages.  Back then, the two languages hadn't diverged as much as they have today.  English is a quickly changing language.