Fast way to enumerate forest and build domain trees in C#?

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C#

active-directory

I need to build a trees of domains in a forest. So far I have used the following code:

using System.DirectoryServices.ActiveDirectory;

...

List<Domain> domains = new List<Domain>(); // flat list of domains
foreach (Domain dom in Forest.GetCurrentForest().Domains)
{
    try
    {
        domains.Add(dom);
        var de = dom.GetDirectoryEntry();
        // Here I have de.Guid and de.Parent.Guid, so using them I can build a tree
    }
    catch () {} // for simplicity left it blank.
}

The code works well. But when some of the domain becomes unavailable, the response (Domain.GetDirectoryEntry()) can take a while (15-20 sec.) before exception throwing. But using Active Directory Domains and Trusts, it is possible to retrieve all forest trees pretty fast with problem domains, so I suppose that there is some cache of domains somewhere.

Q:

If this cache is in the Global Catalog, I wonder - is it possible to have AD structure without GC?

And how can I perform a faster enumeration of forest domains, and build a tree structure?