Joomla 1.5.10 serious performance issues?-Collection of common programming errors

We have build a Intranet on Joomla, its used by more than 20,000 users. At times there are 200 or more concurrent users on the site. And site starts working slow and sometimes crashes. What should we be looking at. Are there known joomla performance issues that can be handled. We doubt that there are some database queries that makes it slow. Any suggestions in this direction will be helpful