Why is a second taking longer than a second? - google-apps-script

Question

I'm trying to create a timer in Google Apps Script, such that when there is a number displayed on a cell, and I start the script, the script will automatically decrement that number once every second, until the value reaches zero.
So I found this answer on a related question, and managed to implement code that does the same thing just fine. My one problem is that the code seems to run slowly - so that a second doesn't actually take a second. At a guess, it's between 1.5 to 2 seconds. This obviously isn't hugely ideal for a timer, and I'm wondering if it's a fixable problem. Is there a code optimization that might make my timer run on time, or something I've not factored in?
Here's a link to a test spreadsheet, with the code in the Script Editor (it's View Only for safety, but the spreadsheet can be copied.) A UI control should initiate the "Start Timer" function if you want to test it.
Here's the full code:
var ss = SpreadsheetApp.getActiveSpreadsheet();
function onOpen() {
var ui = SpreadsheetApp.getUi();
ui.createMenu("Timer controls")
.addItem("Start Timer", "startTimer")
.addToUi();
}
function startTimer() {
var timer = ss.getRangeByName("Timer");
var timeval = timer.getValue();
while (timeval > 0) {
timeval--;
timer.setValue(timeval);
SpreadsheetApp.flush();
Utilities.sleep(1000);
}
}

Answer 1

Timer
function undertest() {
var start=new Date().getTime();//milliseconds
//all your other code here
Logger.log(Number((new Date().getTime()-start)/1000).toFixed(2));//seconds
}

Answer 2

Tl;Dr There is no fix for your code.
You are missing that there are communications processes between your web browser and Google servers...
and that Google Apps Script methods are slow,
and that editing a cell value triggers the recalculation of the whole spreadsheet.
You should start over and instead of using sever side code and Google Sheets to display a timer, follow the recommendation of Cooper on his answer to the same question of the answer that in linked in the question: Use client-side code.