Cells.find Function Slower Than Expected Across Several Worksheets

I wrote a custom find function to search for a list of numbers across a bunch of worksheets (6 in total). A cell reference is sent to the function and it basically returns the worksheet name and address for the first occurence, or nothing if not found.

Now I realise that the number of cells that need to be checked is fairly severe across 6 worksheets (finding 65 numbers * 65536 * 256), but this still takes about 1 minute to execute... is that about right? Code was hacked together quickly and is nothing special...

Public Function My_Find(Optional my_range As Range) As String

Dim ws As Worksheet
Dim my_cell As Variant
Dim result As Variant

Application. ScreenUpdating = False
Application.Calculation = xlCalculationManual