gnus fetching headers for ages

[.Martin.]

Hi all,
When I check for new messages by pressing 'g' and entering a particular
group, the speed is ok, however, when I go back to the list of groups
(Shift + L) and press RET to see old messages from a particular group it
takes ages to enter a particular group. First it is 'Fetching headers
for alt.......' then 'Generating summary'. Obviously it's a waste of
time and bandwith to do it every single time. How can I modify my
configuration to change this behaviour? This is my .gnus

,----
| (setq gnus-posting-styles
|         '((".*"
| ;;              (address "xtd8865@gomain.com")
|         (organization "abc")
|         (signature-file "~/.gnus-sig")
|         ))
| )
| 
| ;; my mail headers - if not using posting styles
| (setq user-full-name ".Martin.")
| (setq user-mail-address "xtd8865@gmail.com")
| 
| (setq   gnus-check-new-newsgroups nil
|         gnus-read-active-file nil
|         gnus-nov-is-evil nil)
| 
| 
| ;; don't ask how many email to dowload
| (setq gnus-large-newsgroup 'nil)
`----

Thanks for your time reading it.
-- 
regards
Martin