As i checked your board yesterday, there where about 150 anonymous users online. You can try to decrease the default thread per page and posts per page setting.
It will only affect anonymous users and newly registered users (who can adjust this setting in their profile).
It should decrease the load.
Another bottleneck could be the search. A search over all postings can bring down the hole board, especially if you have a many postings.
Maybe you can log the memory and cpu usage. Logging the search will also help.
