menu_tree_check_access issue? 9 second page loads

Hello,

Need help with a Drupal 7.5.4 site, Apache on Ubunu - using Boost for anonymous access, but for logged-in users or excluded pages every page load takes 9 seconds - consistently. Turning on devel options and looking at the query log, I am seeing something that doesn't look right and may be causal.

In menu_tree_check_access, every page load produces this query with thousands of arguments:

SELECT n.nid AS nid FROM node n WHERE (n.status = :db_condition_placeholder_0) AND (n.nid IN (:db_condition_placeholder_1, :db_condition_placeholder_2, :db_condition_placeholder_3, :db_condition_placeholder_4, :db_condition_placeholder_5, :db_condition_placeholder_6, :db_condition_placeholder_7, :db_condition_placeholder_8, :db_condition_placeholder_9, :db_condition_placeholder_10, .....

...repeated in this pattern up to...

:db_condition_placeholder_9458))

Any ideas?

Thank you!