Database hitting is slow when my wpf application is idle for 10 to 20 minutes

Please give examples of your query. Have you considered using a stored procedure for the query? The symptoms sound like a query plan being cached and then dropped for a complex query. With a stored procedure the query plan will be held in cache more reliably than an ad hoc query. You can also read up on query hints to short circuit some plan decisions and SQL query performance troubleshooting. A rule of thumb: the more indexes you have, the faster the reads and the slower the writes. Index all fields where you are filtering or joining and use transactions if this affects batch writes.