icloud – Apple Mail retains downloading messages

I’m utilizing a MacBook Air M1 (2020) with Sequoia 15.0 (24A335) and Mail Model 16.0 (3818.100.11.1.3). I exploit the Apple Mail software with accounts from iCloud, Outlook, and Google.

For the previous 10 days, the Mail app has not been working correctly:

1. It retains downloading messages, particularly from Gmail. Once in a while, when opening the “Flagged” folder, a lot of the flagged emails from these accounts merely disappear. After closing and opening the app just a few instances, it begins downloading them once more (hundreds of emails).
2. Even when the messages don’t disappear, downloading/receiving new emails takes a substantial period of time, particularly (maybe solely) from the Gmail accounts.

I attempted to rebuild the mailboxes a number of instances, and in addition eradicating and including the accounts, however nothing fixes the issue.