mh-junk-background

mh-junk-background is a variable defined in `mh-e.el'.
Its value is

nil

Documentation:
If on, spam programs are run in background.

By default, the programs are run in the foreground, but this can
be slow when junking large numbers of messages. If you have
enough memory or don't junk that many messages at the same time,
you might try turning on this option.

Note that this option is used as the "display" argument in the
call to `call-process'. Therefore, turning on this option means
setting its value to "0". You can also set its value to t to
direct the programs' output to the "*MH-E Log*" buffer; this
may be useful for debugging.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 8.0 of the MH-E package that is part of Emacs 22.1.