As I mentioned earlier in a post, causes of slow logons can be many things and troubleshooting this is often time consuming. I recommend you to download the ADK tools for Win8 (for the moment it’s for the Consumer preview) and use the “Windows Performance Recorder” and “Windows Performance Analyzer” to help you find the culprit.

To get an overview of the tool and some examples, take a look at this excellent TechEd session: How many coffees can you drink while Windows 7 boots?