How can we speed up the admin area for WP multiuser? (note: we have 2000 sites on our network).

When we login as a user for one site, the admin area of WP multi user is fast (2 – 3 seconds between pages). But when we login as super admin it seems that loading all 2000 sites to the header utility bar slows things down (13 – 15 seconds between pages).

We use premium hosting company: Rackspace.

Any ideas on how to speed up performance when logged is a super user?