Mac Mail stuck moving messages

I have seven accounts in Apple mail on my MacBook Pro M1 with ~150k messages between them. Four of the accounts are MS O365 Exchange accounts (work-related); two are Fastmail (IMAP) accounts, and one is Gmail. Although I do have an Apple (iCloud) account, I do not access it via Mac Mail.app. I also make extensive use of the SmallCubed MailTags plugin.

All of this makes accessing mail very slow at times, even for the Apple M1 MBP. At the moment, it is has been stuck “moving message 23 of 62” for some considerable time. Advice from various forums suggests this is an iCloud synchronization issue . If that is so, why is iCloud involved? It should be a simple direct client (mac)/server (MTA) communication. It is not helped by Apple’s insistence on downloading and indexing every last message. It should be possible for users to choose which mailboxes are downloaded and index. Most IMAP/Exchange servers have their own search facility, and also a web portal so there are alternative approaches when retrieval of archival, rarely accessed, messages is required. It should be noted that the existing indexing (and thereby searching) is far from perfect, frequently requiring time-consuming mailbox rebuilding. Also, for a complex installation such as mine, the common advice to delete the indexes in ~/Library and allow them to rebuild takes hours and doesn’t actually solve the problem.

If iCloud is involved, I am in the UK. Is it possible that transatlantic synchronisation is inherently slow? It might explain some other issues if it is.