Google Apps Script入門
スプレッドシートが非常に遅い、高速化するには

function mySample35_1() {
  var sheet = SpreadsheetApp.getActiveSheet();
  var tannka,suuryou
  var lastRow = sheet.getLastRow();
  for (i=2; i<=lastRow; i++) {
    tannka = sheet.getRange(i, 2).getValue();
    suuryou = sheet.getRange(i, 3).getValue();
    sheet.getRange(i, 4).setValue(tannka * suuryou) ;
  }
}

function mySample35_2() {
  var sheet = SpreadsheetApp.getActiveSheet();
  var tannka,suuryou
  var lastRow = sheet.getLastRow();
  var ary1 = sheet.getRange(2, 2, lastRow-1, 3).getValues();
  for (i=0; i<=ary1.length-1; i++) {
    ary1[i][2] = ary1[i][0] * ary1[i][1]
  }
  sheet.getRange(2, 2, lastRow-1, 3).setValues(ary1);
}