Windows Live Mail – creating a new blank email message takes as much as 5 minutes

I am running Windows Live Mail under Windows 10 Home (version 1607, OS Build 14393.693) on a desktop PC, details as follows:

"Version 2012 (Build 16.4.3528.0331)

Microsoft® Office 2010 Proofing Tools
© 2010 Microsoft Corporation.  All rights reserved
...
English
Portions of International CorrectSpell™ spelling correction system © 1993 by Lernout & Hauspie Speech Products N.V.  All rights reserved.
Encarta ® World English Dictionary © ℗ 1998 - 2008 Microsoft Corporation. Developed for Microsoft by Bloomsbury Publishing Plc.
The American Heritage® Dictionary of the English Language, Third Edition Copyright © 1992 Houghton Mifflin Company.  Electronic version licensed from Lernout & Hauspie Speech Products N.V.  All rights reserved."

Everything has been fine for 6 months but I now find that when I click on 'Email Message' (to create a blank new email), 'Reply' or 'Reply to All' in the Windows Live Mail client, it becomes unresponsive for about 3-5 minutes before finally opening the new window for the new email.

That sort of delay makes the client unusable for sending emails.

I know that Windows Live Mail is being replaced but it should still work, so I understand.

Any ideas what the problem might be or what I need to do to cure it?