gnus-updated-mode-lines

gnus-updated-mode-lines is a variable defined in `gnus.el'.
Its value is

(group article summary tree)

Documentation:
List of buffers that should update their mode lines.
The list may contain the symbols `group', `article', `tree' and
`summary'. If the corresponding symbol is present, Gnus will keep
that mode line updated with information that may be pertinent.
If this variable is nil, screen refresh may be quicker.

You can customize this variable.