I noticed this problem, too. i think the time come to take hand the db management of the forum.
You can hire DB admins and use professional db solutions. So many people use the forum and stability is important. Sphinx http://sphinxsearch.com/ does great success on loads if you know it. And all db queries has to be optimized. I don't know how you cluster but  amazon has great clustering services too but you need to able to manage it nicely.