It takes a long time to get and display an active file with Gnus.

I did a little profiling.  These number are for showing the active file
of my local news server twice (it has ca. 48.000 groups):

Function Name                              Call Count  Elapsed Time  Average Time
=========================================  ==========  ============  ============
gnus-server-read-server                    2           199.092545    99.5462725
gnus-browse-foreign-server                 2           199.086051    99.5430255
gnus-group-prefixed-name                   97248       38.465528999  0.0003955405
gnus-server-equal                          97248       26.304658999  0.0002704904
gnus-add-text-properties                   97390       9.0370400000  9.279...e-05
gnus-group-level                           97248       7.4187510000  7.628...e-05
gnus-group-name-decode                     97432       6.9541069999  7.137...e-05
gnus-group-name-charset                    97340       6.8508260000  7.038...e-05
gnus-request-list                          2           6.579343      3.2896715
nntp-request-list                          2           6.579015      3.2895075
nntp-send-command-and-decode               2           6.5783070000  3.2891535000
nntp-accept-process-output                 31          4.5154390000  0.1456593225
gnus-server-to-method                      97320       3.2123899999  3.300...e-05

A lot of time is spent in `gnus-group-prefixed-name' because it's called
on every iteration in the inner loop.  This shouldn't be necessary,
because I think the prefix is the same for every group on the server(?).

So, I tried to move it outside of the loop [1], which turns out to be
around 20% faster:

Function Name                              Call Count  Elapsed Time  Average Time
=========================================  ==========  ============  ============
gnus-server-read-server                    2           154.793711    77.3968555
gnus-browse-foreign-server                 2           154.78735     77.393675
gnus-add-text-properties                   97392       9.8606260000  0.0001012467
gnus-group-name-charset                    97342       9.2035850000  9.454...e-05
gnus-request-list                          2           6.51363       3.256815
nntp-request-list                          2           6.513376      3.256688
nntp-send-command-and-decode               2           6.5125790000  3.2562895000
gnus-group-name-decode                     97436       5.2722589999  5.410...e-05
nntp-accept-process-output                 33          4.3882959999  0.1329786666
gnus-group-level                           97248       4.3642049999  4.487...e-05


It's probably not a totally clean way of doing it, so I don't know if
the speed improvement is worth it.

[1]