The collection processes the sub folders. If you run the following macro, which uses the same process, in Outlook it will list all the sub folders for the selected folder in the immediate window of the VBA editor (Ctrl+G). This will be much faster than the mail count.
Sub ListFolders()
Dim cFolders As Collection
Dim olFolder As Outlook.Folder
Dim SubFolder As Folder
Dim olNS As Outlook.NameSpace
Set cFolders = New Collection
Set olNS = GetNamespace("MAPI")
cFolders.Add olNS.PickFolder
MsgBox "This could take a while to process!"
Do While cFolders.Count > 0
Set olFolder = cFolders(1)
cFolders.Remove 1
Debug.Print olFolder.Name
For Each SubFolder In olFolder.folders
cFolders.Add SubFolder
Next SubFolder
DoEvents
Loop
Set cFolders = Nothing
Set olFolder = Nothing
Set SubFolder = Nothing
Set olNS = Nothing
End Sub