How do I copy old transactions to a recoverable file?

What leads you to believe the account is slowing down your backup?  Have you compared the performance a backup of your data file with and without the account?

The obvious way of preserving the current data is to save backup (press Ctrl + B).  If you really want to preserve the account's transactions in an importable format, I suggest you export the account in QIF format: https://www.quicken.com/support/how-do-i-export-data-quicken