ComboBoxes Collection?

I am using the following code to determine whether a given range is the linked cell for a Combo Box by looping through the shapes collection.

Function LocateFormControl(OverRange As Range) As Shape
Dim objTemp As Shape
For Each objTemp In OverRange.Parent.Shapes
If Left(objTemp.name, 6) = "Drop D" Then
If WorksheetFunction.Substitute(objTemp.ControlFormat.linkedcell, "$", "") = WorksheetFunction.Substitute(OverRange.Address, "$", "") Then
Set LocateFormControl = objTemp
Exit Function
End If
End If
Next
Set LocateFormControl = Nothing
End Function

However, when I use this code on big worksheets with many other shape objects (such as Comments), the program runs very slowly. Is there a ComboBoxes Collection that I could use to avoid looping through all shapes on the worksheet, or is there a different, faster way to run this code?