Update - The Add-On continued to slow IE down to a crawl every time I tried it again for a couple of hours or so after I had updated the program to the latest version. Since it seems that Avast updates itself every few hours, I kept trying it. Eventually, sure enough, Avast must have "phoned home" again WITH the newest program version, and that cleared up the problem with the Add-On. I'm just guessing here, but the timing was interesting. So now everything is working as expected. No problems with the Add-On since then. Thanks to everyone for their help!