I was troubleshooting a client's DNN site today because the response time was horrible. Their site was relatively small and every page load was taking up to 30 seconds. By logging in to the production server, I could see that the CPU would spike to 100% on the sqlsrvr.exe process during the entire page load so that gave me a starting point. Then I dug in to SQL Profiler to identify the problematic query. The resolution was to delete everything from the ScheduleHistory table. I purged the data and the website immediately became fast again. If you notice SELECT queries on the ScheduleHistory table taking a long time, this may be the solution to your problem.