Slow first page load on asp.net site
The IIS application pool is shut down after 30 minutes of inactivity. After that, when you make a request IIS basically has to start the website up again, which leads to the behavior you are describing. You can change the idle time of your website in iis though to avoid it.
If you're using IIS 7 there is an IIS plugin call Application Warm Up that will help
I've written a blog post about my experiences using it here (don't know if i'm allowed to post to my own content?):
http://www.diaryofaninja.com/blog/2010/05/06/keep-your-aspnet-websites-warm-and-fast-247