Build Union of even lines with a fast approach based on divide and conquer.

gistfile1.cs

 private static Range UnionDivideAndConquer(int[] excelRows, int start, int end)
{
  if (excelRows.Length == 0) return null;
  if (start == end)
  {
      return ExcelAddIn.ActiveSheet.Range["A" + excelRows[start] + ":" + "C" + excelRows[start]];
  }
  int middle = (start + end) / 2;
  Range left = UnionDivideAndConquer(excelRows, start, middle);
  Range right = UnionDivideAndConquer(excelRows, middle + 1, end);

  return ExcelAddIn.Application.Union(left, right);
}

public BuildUnionFast(int count)
{
  var rowIndices = Enumerable.Range(1, 2*count+1).Where(k => k % 2 == 1).ToArray();
  Range range = UnionDivideAndConquer(rowIndices, 0, rowIndices.Length-1);
}