Archive To-Do Lists

As a Microsoft Outlook user since 1999, I have accumulated significant to-dos - all cataloged by year under one group [History] in the To-Do App.

Unfortunately, the sheer volume of these to-do's has made syncing on the iPhone and Mac iOS (apps) cumbersome and very slow. How may I transfer the prior years [History] to an archive where the syncing becomes more agile and yet allows me to search for historical to-do's if needed?

Thank you in advance for any guidance.