Poor performance when deleting questions on large sites, > 250000 questions

We have upgraded our production 1.8.4+ site on a test install to "1.9 + (Build: 20080312) (2007101509)". We have over 250,000 questions and over 6000 quizzes. Some other details:
Solaris 10 and Coolstack 1.1
We have separate web and db servers.
Courses: 13299
Users: 24834
Role assignments: 154804
Teachers: 1402
Questions: 264090

Deleting questions has a roughly linear behavior, taking 7 seconds per question when selecting delete from the question bank (/question/edit.php). It appears to be limited completely on the DB side as there is no change in CPU load on the web server but significant change in CPU load on the DB.

The performance has improved over 1.8. The confirmation step "Are you absolutely sure you want to delete the following questions?", is <<1 sec, while it takes a significant amount of time on 1.8. The CPU load on the DB is less.

Can the queries when checking wether questions are used in a quiz be improved in 1.9 or is it a case of beef-up/tune the DB server? What information can we provide to help?