Add Rows and copy formulas from Row before - Timeout - google-sheets

Question

I have a large sheet with over 4000 rows and 30 columns.
I am trying to automaticly add new rows to the sheet if there are less than x "empty" rows. Empty rows are defined by just containing formulas but the data column A is empty.
To check if a row is empty or not I check column A, because you have to enter data in column A. After adding new rows the script should copy/paste the last row before over the new rows, so they contain the formulas as well.
Here is the script (PREVIOUS VERSION):
function addRowsItems() {
var ss = SpreadsheetApp.getActive();
var sh = ss.getSheetByName('items');
var freeRows = 300; // Number of empty Rows after last Entry
var lRow = sh.getLastRow(), lCol = sh.getLastColumn(), range = sh.getRange(lRow,1,1,lCol);
var startRow = lRow-freeRows;
if(startRow < 0) {
startRow = 1; }
var values = sh.getRange("A" + startRow + ":A").getValues();
var maxIndex = values.reduce(function(maxIndex, row, index) {
return row[0] === "" ? maxIndex : index;
}, 0);
var maxIndex = maxIndex + startRow;
var space = lRow - maxIndex;
var addLines = freeRows - space;
if(space < freeRows) {
sh.insertRowsAfter(lRow, addLines);
range.copyTo(sh.getRange(lRow+1, 1, addLines, lCol), {contentsOnly:false});
}
}
This is working in a new sheet with less data.
But using it in the main sheet with over 4000 rows of data I get a time out. The script adds the new rows but it times out before it is able to copy/paste.
Why is it taking so much time? Is there an easier way to achieve that?
So this is the actual version. Cell B1 contains COUNBLANK of Range A:A.
function addRowsItems() {
var ss = SpreadsheetApp.getActive();
var sh = ss.getSheetByName('items');
var freeRows = 307; // Number of empty Rows after last Entry
var lRow = sh.getMaxRows(), lCol = sh.getMaxColumns(), range = sh.getRange(lRow,1,1,lCol);
var space = sh.getRange("B1").getValues();
var addLines = freeRows - space;
if(space < freeRows) {
sh.insertRowsAfter(lRow, addLines);
range.copyTo(sh.getRange(lRow+1, 1, addLines, lCol), {contentsOnly:false});
}
}

Answer 1

Try the COUNTBLANK function returns a count of empty cells in a range. Cells that contain text, numbers, errors, etc. are not counted. Formulas that return empty text are counted.