Apparently this is a well known performance boost for Apple Mail users – I wish I'd known about it sooner! I'd been having all sorts of problems with performance and Mail.app hanging, and this simple trick has it like new again.
I have to wonder why such a thing isn't done by Mail.app itself given how bad it did get over the course of the last 6 months.
(Via Brett Porter.)
Worked great for me. Pretty amazing little trick!