Gnus slow with IMAP when changing ISP's

[Damian]

Hello,

I use gnus from work and from home. I have a different ISP in each
place. When I start gnus from one of those places (say home) I can
access my IMAP account almost right away, however, if I go to the
other place (my work) checking for new mail takes forever.

In any case the situation is the same. Fire up gnus, read mail,
suspend laptop, go to the other place, wake up laptop, check mail,
wait till hell gets cold.

I've tried restarting gnus, but the problem remains. I have to restart
the whole emacs in order to get the normal behaviour again.

Does anybody have an idea about why this is happening?

Thank you in advance,
Damian.