Pass from a for cicle to a search cicle

To increase performance I would like to optimize the following macro. Probably passing from a for cicle to a search.
Any suggestion?

Dim DataRange As Variant
Dim Irow As Long
Dim Icol As Integer
Dim MyVar As Double
DataRange = Range("A1:c322000").Value
DataRange1 = Range("J1:J322000").Value
For Y = 1 To 234100
For Irow = 1 To 322000

MyVar = DataRange(Irow, 1)
MyVar2 = DataRange1(Y, 1)
  If MyVar = MyVar2 Then
DataRange(Irow, 3) = "X"
Debug.Print Y
GoTo yy
Else
End If


Next Irow
yy:
Next Y


Range("A1:C1000000").Value = DataRange

End Sub