Slow appdomain shutdown
If you have a lot assemblies loaded in the appdomain, you may experience slow appdomain shutdown, coupled with CPU spike.
This thread explained why.
http://msdn.microsoft.com/newsgroups/default.aspx?dg=microsoft.public.dotnet.framework.performance&mid=f326e1d1-00b7-4e0a-90ff-09ae7122947f
