Well - 7000 message take a while to load. I don’t know the code of the Mail app - but maybe it is not caching the message view but always reloads all messages when opening the Inbox. For browser based mail access I prefer Roundcube - that works fine even when opening folders with many messages as it caches everything in its database.
The reason why Thunderbird is faster is the same - Thunderbird does not load the messages it already has. So even if the Inbox has 7000 messages, Thunderbird only loads them once and will later only request new messages when opening a folder since the last time it connected to the server.
Anyone has some input as why the Mail app can’t display the inbox that has 2 times less eMails and GB of Data than my Sent items Folder that can be displayed without problems ?