Problems Loading Emails in Apple Mail

Using Apple Mail Ver 16.0 with macOS Sonoma Ver 14.4.1. When opening the Mail program and selecting emails under an iCloud email address there is a delay in them being displayed, with 'Downloading' being displayed on the screen. This applies regardless of the length of the email or if it has attachments or not. It happens whenever the program is first opened.

I have a second iCloud email address associated with another Apple ID account and also an account with another email address (@xtra.co.nz) and there are no problems with these accounts.