how can I keep gnus from Hiding All Threads when open a group

[Harry Putnam]

I notice when I open a group, gnus hides all threads seemingly
automatically.

But when opening really large groups; 12,000 and up, even 50,000 msgs
that process of hiding all threads can become a hefty time sink.

Even when unplugged and all the messages are already downloaded.

How could I go about telling gnus to leave the threads unhidden or
would that have the effect I'm hoping for... i.e. speeding up opening
really large groups.