From the Gnus FAQ (see Reading news with Emacs):
If you have a slow machine, or are just really impatient, there are a few things you can do to make Gnus run faster.Set
gnus-check-new-newsgroupsandgnus-check-bogus-newsgroupstonilto make startup faster.Set
gnus-show-threads,gnus-use-cross-referenceandgnus-nov-is-eviltonilto make entering and exiting the summary buffer faster.