As an experiment which wouldn't hurt much (certainly not as much as the current slowdowns), I second the plan to disable the user contributions feature for now.
Jason or Jimbo, the quickest and dirtiest way to do that would be to remove/rename the special_contributions.php file.
As mentioned earlier, it would also be good to remove/rename special_asksql.php because the developer passwords have been compromised. It is even possible that some of the direct sql queries are slowing down the site.
Axel