Serialization Suddenly Slow - How To Fix It

I've been cloning my objects using serialization for awhile, and typically its been quick, at least for my purposes. After a series of updates to these objects, however, now the serialization process is much slower. Trial-and-error has so far failed to pinpoint the cause. Some of the added elements to my object are:

1.binding lists

2. an inherited dictionary class

3. more internal references (the object is actually group of nested objects, and these internal objects have more references to each other than before)

I have been careful to avoid/eliminate references to external elements when cloning, as that can lead to a cascade of extra cloning. So I think the problem is with the object structure itself. This is my serialization routine:

Friend Function CloneObj(ByVal obj As Object) As Object
If Not obj.GetType.IsSerializable Then Throw New Exception("Object not serializable")
Dim bf As New System.Runtime.Serialization.Formatters.Binary.BinaryFormatter

[code]....