To diagnose problems with Thunderbird, try one of the following:

• Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.
• Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 10, Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

None of the steps above solved the problem. So, I am looking into more arcane issues. I can still address emails by the cut and copy method, but it is a bit clumsy doing so. I thank you for your help, Max.

This is not an answer to your slow performance.

If you enable the Contacts Sidebar in the Compose window you can add Contacts from that. It should be less painful than the copy-and-pasting that you allude to. It would also be interesting to see if this is also slow, since it too is accessing the Address Book.

F9, or View|Contacts Sidebar in the menu.

Thanks, Zenos, that helps. I have a Quirky Computer. Today the address line accepts a full address!