gnus-read-active-file

gnus-read-active-file is a variable defined in `gnus-start.el'.
Its value is

some

Documentation:
*Non-nil means that Gnus will read the entire active file at startup.
If this variable is nil, Gnus will only know about the groups in your
`.newsrc' file.

If this variable is `some', Gnus will try to only read the relevant
parts of the active file from the server. Not all servers support
this, and it might be quite slow with other servers, but this should
generally be faster than both the t and nil value.

If you set this variable to nil or `some', you probably still want to
be told about new newsgroups that arrive. To do that, set
`gnus-check-new-newsgroups' to `ask-server'. This may not work
properly with all servers.

You can customize this variable.