When Gnus starts, or indeed whenever it tries to determine whether new articles have arrived, it reads the active file. This is a very large file that lists all the active groups and articles on the server.
Before examining the active file, Gnus deletes all lines that match the
regexp gnus-ignored-newsgroups. This is done primarily to reject
any groups with bogus names, but you can use this variable to make Gnus
ignore hierarchies you aren’t ever interested in. However, this is not
recommended. In fact, it’s highly discouraged. Instead, see New Groups for an overview of other variables that can be used instead.
The active file can be rather Huge, so if you have a slow network, you
can set gnus-read-active-file to nil to prevent Gnus from
reading the active file. This variable is some by default.
Gnus will try to make do by getting information just on the groups that you actually subscribe to.
Note that if you subscribe to lots and lots of groups, setting this
variable to nil will probably make Gnus slower, not faster. At
present, having this variable nil will slow Gnus down
considerably, unless you read news over a 2400 baud modem.
This variable can also have the value some. Gnus will then
attempt to read active info only on the subscribed groups. On some
servers this is quite fast (on sparkling, brand new INN servers that
support the LIST ACTIVE group command), on others this isn’t fast
at all. In any case, some should be faster than nil, and
is certainly faster than t over slow lines.
Some news servers (old versions of Leafnode and old versions of INN, for
instance) do not support the LIST ACTIVE group. For these
servers, nil is probably the most efficient value for this
variable.
If this variable is nil, Gnus will ask for group info in total
lock-step, which isn’t very fast. If it is some and you use an
NNTP server, Gnus will pump out commands as fast as it can, and
read all the replies in one swoop. This will normally result in better
performance, but if the server does not support the aforementioned
LIST ACTIVE group command, this isn’t very nice to the server.
If you think that starting up Gnus takes too long, try all the three different values for this variable and see what works best for you.
In any case, if you use some or nil, you should definitely
kill all groups that you aren’t interested in to speed things up.
Note that this variable also affects active file retrieval from secondary select methods.