My guess is that the AV downloads and scans the mail and sends a few “bites” every now and then to the mail client so that this sees some progress and does not time out.
The behavior is exactely as I had it with bitdefender ages ago.
not checked yet, but my assumption is yes, because some mails seem to come in “like normal” and mails with known attachments also come in without delay.
Yes, I’m sure it’s a little bug that causes some loop or so to run falsely. Something stupidly trivial like a double var that is uses 32 bit on a 32 bit OS and 64 bit on a 64 bit OS and they did not check and catch an overflow of the 32 bit var as they only work on 64 bit systems where it never happens… something like that.