SSRS Reports are taking too much time to load.

I use a Windows Application where generating reports taking too much time. The underlying queries are not changed. This issue is encountered recently which usually takes 10 seconds to load but now it is loading in 5 minutes approximately. The issue gets resolved whenever I do an IISRESET on the Application server. But the issue is repeated every alternate day. Every time I need to do an IISRESET. To solve this temporarily I created a Windows Scheduled task which runs every day to do an IISRESET. But no help. The issue is still encountered. I need to know the root cause of the issue and a possible resolution.