Android Question File.Copy2Async with ExternalStorage is very slow and blocks the app (isn't responding)

            In = File.OpenInput(File.DirInternal & "/temp", f)
            LFile=storage.FindFile(Lfolder,f)
            If LFile.Length > 0 Then Storage.DeleteFile(LFile) 'without deleting sometimes corrupt files are created
            LFile=Storage.CreateNewFile(Lfolder,f)
            out=Storage.OpenOutputstream(LFile)
            Wait For (File.Copy2Async(In, out)) Complete (Success As Boolean)

Sub copy
    Dim Inp As InputStream
    Dim out As OutputStream
    Dim externalfilelist As List
    Dim Lfolder As ExternalFile
    Lfolder=Storage.Root
    externalfilelist.Initialize
    ListFiles (Lfolder) 'create a list "lstext" of all files in external storage
    For Each f As String In File.ListFiles(File.DirInternal & "/temp")
        If File.Exists(File.DirInternal & "/temp", f) Then
                Inp = File.OpenInput(File.DirInternal & "/temp", f)
                Dim test As Int = externFileListTest(f) 'check if the file exists in the list
                If test > -1 Then Storage.DeleteFile(externalfilelist.Get(test)) 'if the file exists in the list, delete it.
                out= Storage.OpenOutputstream(Storage.CreateNewFile(Lfolder,f))
                Wait For (File.Copy2Async(Inp, out)) Complete (Success As Boolean)
                File.Delete(File.DirInternal & "/temp", f)
                out.close
        End If
    Next
End Sub

Public Sub ListFiles (Folder As ExternalFile)
    Dim files() As Object = Folder.Native.RunMethod("listFiles", Null)
    For Each o As Object In files
        Dim f As JavaObject = o
        externalfilelist.Add(DocumentFileToExternalFile(f))
    Next
End Sub

Private Sub DocumentFileToExternalFile (DocumentFile As JavaObject) As ExternalFile
    Dim ef As ExternalFile
    If DocumentFile.IsInitialized = False Then
        Return ef
    End If
    ef.Initialize
    ef.Name = DocumentFile.RunMethod("getName", Null)
'    ef.Length = DocumentFile.RunMethod("length", Null)
'    ef.IsFolder = DocumentFile.RunMethod("isDirectory", Null)
    ef.Native = DocumentFile
'    ef.LastModified = DocumentFile.RunMethod("lastModified", Null)
    Return ef
End Sub

Sub externFileListTest(text As String) As Int
    Dim Test As ExternalFile
    For i = 0 To externalfilelist.Size - 1
        Test = externalfilelist.Get(i)
        If Test.Name = text Then
            Return(i)
        End If
    Next
    Return(-1)
End Sub