nnimap: Server buffer vs secondary-select-methods

[Mark Plaksin <happy@mcplaksin.org>]

Checking nnimap groups added via the Server buffer is very slow unless you
also add them to gnus-secondary-select-methods.  When groups are added via
Server, Gnus opens a new IMAP connection each time it checks them.  I'm
using CVS Gnus updated today.

For example, I added a brand-new IMAP server via the Server buffer:
(nnimap "example"
        (nnimap-address "example.com")
        (nnimap-server-port 993)
        (nnimap-stream ssl)
        (nnimap-list-pattern
         ("INBOX" "Mail/*")))

Then I hit SPC on the new server and subscribed to a group.  After that,
whenever I g in the Summary buffer, Gnus will open a new connection to
example.com to check the folder.  It opens a connection every time I hit
g.

If I add the server to gnus-secondary-select-methods, Gnus opens the
connection once the first time it checks the folder and reuses the
connection after that.

Is there a reason for this behavior?  Surely I'm missing something!
Looking at the code, before the server is in gnus-secondary-select-methods
nnimap-server-opened decides the server isn't open because this
nnoo-server-opened call says it's not: 

  (nnoo-server-opened 'nnimap (or server nnimap-current-server))

I have always struggled to understand the difference between using the
Server buffer and using gnus-secondary-select-methods.  Today I made Gnus a
zillion times (no exaggeration :) faster by finally putting my IMAP servers
into gnus-secondary-select-methods.