I am using the macro below in order to compare two lists at column A and B
But the code is too slow
it takes 2 hours to complete (pentium IIII 2.4 GHz)
is there any idea for a faster way of comparing lists with 20.000 data in a
row?

(The code delete any content at column A if find it at column A)

Sub Comparing()
Dim A As Range, B As Range, C As Range
Dim i%, y%, z%
Set A = Columns(1)
Set B = Columns(2)
i = 1: y = 1: z = 1
Do Until IsEmpty(A.Cells(i))
Do Until IsEmpty(B.Cells(y))
If A.Cells(i) = B.Cells(y) Then
A.Cells(i).ClearContents
z = z + 1: y = 1
Exit Do
End If
y = y + 1
Loop
i = i + 1: y = 1
Loop
End Sub