Writing data in Excel file using JavaScript is resulting in performance issue.

There are things you can reduce the performance issue by using OleObject

1. Store values in the Project Variable table type 
2. End of the run create a function that can covert Project Variable into the Excel Application

For Example:

function WriteExcelDataToPV(pvName,rowCell,columnCell,value){
    Project.Variables.VariableByName(pvName).$set("Item", rowCell, columnCell, value);
}  
//Converting pv to Excel
function convertPVToExcel(strexcelFileName,pvName){
 var excelObject, excelWorkbookObj,excelMainSheet;
 
 try
 {
 var pv = Project.Variables.VariableByName(pvName);
 
 excelObject = Sys.OleObject("Excel.Application");
 
 excelObject.Visible = false;
 excelObject.DisplayAlerts = false;
 
 excelWorkbookObj = excelObject.Workbooks.Add();
 excelWorkbookObj.SaveAs(strexcelFileName);
 excelWorkbookObj.Close();
 excelWorkbookObj = null;
 
 excelWorkbookObj = excelObject.Workbooks.Open(strexcelFileName);
 excelMainSheet = excelWorkbookObj.Sheets.Item("Sheet1");
 excelMainSheet.Name = "Data";
 //adding columns
 for(var i = 0 ; pv.ColumnCount ; i++){
 excelMainSheet.Cells(0,i).Value = pv.ColumnName(i);
 }
 //adding row data
 for(var r = 0 ; r < pv.RowCount ; pv++){
 for(var i = 0 ; pv.ColumnCount ; i++){
 excelMainSheet.Cells(r,i).Value = pv.ColumnName(i);
 }
 }
 excelWorkbookObj.Save();
 excelWorkbookObj.Close();
 }
 catch(ex)
 {
 Log.Error("Not able to create excel file. Error Description: " + ex.stack); 
 }
 finally
 {
 excelObject.Visible = true;
 excelObject.DisplayAlerts = true;
 excelObject.Quit()
 //CleaningUp
 excelObject = null;
 excelWorkbookObj = null;
 excelMainSheet = null; 
 }
}