I've got GlassFish running as a service on a Windows Server (2008 R2). If you're actively using GlassFish, it stays nice and fast. However, if you go away and don't use it for several minutes, when you come back, any Glassfish activity (app or admin panel) takes around 60 seconds to start up. During that delay period, the server is showing completely idle, as if it hasn't noticed the request yet. At the end of the time, there's a quick processor spike as GlassFish handles the request.
Not sure where else to go with debugging this problem. It wasn't like this when originally installed. Happened sometime in the last few months. Hard to say when as I only use it rarely, and the end users are overly tolerant of slow computers. :-)