I ended up removing your plugin from the IVE PTA site as it was causing the home page to take more than 10 seconds to load, even with the new beta version you sent me. Just taking to long to try to grab that feed that is not conforming to what simple-pie expects. I found another simple plugin that works for the modest needs of that web site, Mailchimp Campaign Archive... the shortcode for that works in widget section also, and doesn't seem to slow the site down at all.
Your plugin still works great on my Music-And-Technology.com site, which is pulling in and consolidating several industry feeds... no slow downs there. So, just something about that Mailchimp list archive that yours doesn't like and makes it slow way down.
Thanks for all the speedy help, though! Very much appreciated!