If there is no activity recorded in the standard shield detailed view then it isn't scanning, as you are basically indicating when you say "The Avast icon rotates once when I select the message but doesn't move during the 17+ seconds of disk activity."
msimn (MicroSoft Internet Mail and News, OE) CPU activity has nothing to do with avast the CPU activity would be on either ashMaiSv.exe or ashServ.exe if it was avast scanning.
Baffling is right - Facts differ, for me I have absolutely no delay with OE6 and avast, if this were the case that there was a delay with OE and avast there would be many posts on the forums and yours is the first I have seen in nearly 4 years.
So I'm at a loss as to why you have. You said that you have ZA you didn't say what version, might this have an email module (I haven't used ZA in many, many years, so I'm not familiar with the latest versions).
That is the only thing I can think of is conflict with something else and when one half of a conflict is removed, no conflict, but doesn't say what is responsible for the conflict.