Regression: email sync & body download are painfully slow

### What Happened?

I upgraded from Elementary 5.1 to 6.1 and I immediately notice performance issues in the Mail app:

1. After adding a Gmail account I thought the Mail application didn't work: the counter was up to date, but the inbox was empty? No message, no loader, nothing. Just empty. It took so long that I went to do other things until I came back to see emails finally did appear.

2. When opening an email, they often don't have a body. I first thought they had no body, only to notice the next day that they did have a body, it just took many minutes to finally appear.

3. The unread state seems to be changed at random interval (immediate, or some seconds or lots of time has passed). It doesn't depend on the body being displayed or not.

### Steps to Reproduce

1. Add a Gmail account with lots of email
2. See Mail display the account name
3. click inbox (unread counter is up to date)
4. the inbox is empty for _many_ minutes

### Expected Behavior

1. The inbox should show the first dozen emails (or so), and at least show a loader to indicate that it's loading.
2. Clicking an email should show the body _quickly_ or at least displays a loader.
3. Changing the unread state should be immediate or a sensible and regular interval (e.g. 3 seconds).

### OS Version

6.x (Odin)

### Software Version

Latest release (I have run all updates)

### Log Output

_No response_

### Hardware Info

Intel i7 i7-4712HQ (haswell, quad-core), 16GB of memory. Plenty fast. Fresh and bare Elementary 6.1 install (empty disk, empty home, didn't even install anything, yet).

Those issues never happened in Elementary 5. Geary also has no issues, albeit it takes a split second to display the email body (not several seconds or minutes).