for Y := 0 to ABitmap.Height - 1 do
    begin
      P0 := ABitmap.ScanLine[Y]; // вычисляем на каждой итерации
      //......
    end

  procedure BitmapDarkness(Bitmap: TBitmap; Value: Integer);
  var
    BitmapDelta: Integer;
    BitmapPixels: Integer;
    X, Y: Integer;
    Dest: PRGBTriple;
  begin
    with Bitmap do // вычисляем один раз
    begin
      BitmapPixels := Integer(ScanLine[0]);
      BitmapDelta := Integer(ScanLine[1]);
      Dec(BitmapDelta, BitmapPixels)
    end;
    for Y := 0 to Pred(Bitmap.Height) do
    begin
      Dest := PRGBTriple(BitmapPixels + BitmapDelta * Y);
      for X := 0 to Pred(Bitmap.Width) do
      begin
        with Dest^ do
        begin
          rgbtBlue := Limit(rgbtBlue + Value);
          rgbtGreen := Limit(rgbtGreen + Value);
          rgbtRed := Limit(rgbtRed + Value);
        end;
        Inc(Dest)
      end
    end
  end;

function TBitmap.GetScanLine(Row: Integer): Pointer;
begin
  Changing(Self);
  with FImage.FDIB, dsbm, dsbmih do
  begin
    if (Row < 0) or (Row >= bmHeight) then
      InvalidOperation(@SScanLine);
    DIBNeeded;
    GDIFlush;
    if biHeight > 0 then  // bottom-up DIB
      Row := biHeight - Row - 1;
    Result := PByte(bmBits) +
      Row * BytesPerScanline(biWidth, biBitCount, 32);
  end;
end;