gnus-read-all-available-headers

gnus-read-all-available-headers is a variable defined in `gnus-sum.el'.
Its value is

nil

Documentation:
Whether Gnus should parse all headers made available to it.
This is mostly relevant for slow back ends where the user may
wish to widen the summary buffer to include all headers
that were fetched.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.