NFS (v3) exports limited by netgroup crash mountd

I have a netgroup with some 200 or so hosts listed in it.  When I add it 
as an export control on a single file system it slows mountd down a bit 
but not badly.

However, if I add it to more than a few file systems in /etc/exports 
mountd becomes very slow in replying and eventually gives up and dies.

It doesn't matter if the netgroup is obtained out of /etc/netgroup or a 
NIS map.  The behavior is exactly the same either way.

The problem exists on 6.1-STABLE as well as 6.2-PRERELEASE compiled last 
night.

I'd really like to get this working.  The idea of exporting hundreds of 
user home directories without at least limiting what hosts they can talk 
to makes me very nervous.  Yes, I know NFS = No F***ing Security but I'd 
still like to make *some* attempt at reasonableness.  Limiting to 
network isn't sufficient, there are lots of students with laptops 
plugging in everywhere.

Any hints or ideas?

thanks,
nomad