How to improve this macro to run faster

Hi all expert there,

I'm new to this website and so with the macro thing. Recently, I learn on how to use macro in simulating reservoir for hydropower project. I have managed to prepared one but it too time consuming. Can someone out there to check if there is any improvement for my vba below? Your comment is highly appreciated.

Sub Button1_Click()
Dim y, x As Integer
x = Cells(18, 2).Value + 20
For y = 21 To x
        ' First run plant discharge starts at 0.
        Cells(y, 8).Value = 0
        Do While Cells(y, 6).Value >= Cells(7, 5).Value ' Current reservoir level above MOL
            ' Increment @ 5.
            Cells(y, 8).Value = Cells(y, 8).Value + 5
            If Cells(y, 21).Value >= 0.98 * Cells(13, 11).Value Then
            ' almost there.
            Exit Do
            End If
        Loop
        Do While Cells(y, 6).Value >= Cells(7, 5).Value
            Cells(y, 8).Value = Cells(y, 8).Value + 0.1
            If Cells(y, 21).Value >= Cells(13, 11).Value Then
            Cells(y, 21).Value = Cells(y, 21).Value
            ' Rated power is achieved.
            Exit Do
            End If
        Loop
        If Cells(y, 21).Value >= Cells(14, 11).Value Then
            Cells(y, 8).Value = Cells(y, 8).Value
        ElseIf Cells(y, 21).Value < Cells(14, 11).Value Then
            Cells(y, 8).Value = 0
        End If
      
Next y
End Sub

Thanks,

jerry from Malaysia.

Sorry for my bad english.