I am using the code below to place a sum subtotal of values in Column V
whenever there is blank cell in Column U. The macro provides the correct
result but as the sheet has over 20,000 entries the macro takes about 10
minutes to complete. Is there a more time efficient (or non-VBA) way to
provide the same result?

Thanks

Sub test()
'place a subtotal in column(V) wherever column(U) has a blank cell
lrow = Range("u65536").End(xlUp).Row + 1
For Each cell In Range("u2:U" & lrow)
If cell.Value = isblank Then
cell.Offset(0, 1).Formula = "=SUM(R[" & -RowCount & "]C[-1]:R[-1]C[-1])"
RowCount = 0
Else
RowCount = RowCount + 1
End If
Next
End Sub