We recently noticed a slow down in performance for one of our ASP.NET applications and after thoroughly checking for code changes, determined none of our code had changed. So then we started checking for OS updates but nothing new there either. Finally we checked the event log for errors and found several services had died near the same time that our performance degraded. One of those services was the DNS Client service.
Performance instantly improved after we restarted the DNS Client service.
So if your web app slows down and your not sure why make sure to check this service is running.