Hi,
I have earlier read here at the forum that people had big problems with slow loading times when the nopCommerce application haven't been active on the IIS for some time.
Have you tried the new feature for the webconfig of asp.net 4: Autostart?
http://weblogs.asp.net/scottgu/archive/2009/09/15/auto-start-asp-net-applications-vs-2010-and-net-4-0-series.aspx
Joe Stagner also bescribes it very good in this video:
http://www.asp.net/web-forms/videos/net-4/core/aspnet-4-quick-hit-auto-start