Why is range protection taking so long - google-apps-script

Question

I am needing to range protect cells on a sheet based on the name in the cell. I can obtain a persons email address from their name and protect the cell so only they can edit it.
It seems to be taking a very long time to add a person to the range protection. I have used the logger to find out what is taking so long and it appears protection.addEditor() is taking 27700ms and protection.removeEditors() is taking 21904ms. Is this normal?
The workbook is shared with a set group of about 30 people, there are about 60 sheets and the workbook contains about 300,000 cells in total.
Any ideas why this is taking so long?
I am getting an Error: Exception: Service Spreadsheets timed out while accessing document with id .... This does not happen every time the script is run, but when it does it always happens at the point where I am protecting a cell. I run an overnight process to look for new entries and protect those cells so only the named person can edit. It is used as a shift booking system.
I was hoping it might be possible to protect all the cells that need protecting in one go for each sheet, but I cannot find anything about how to do this.
function protectActiveCell() {
var sheet = SpreadsheetApp.getActiveSpreadsheet().getActiveSheet();
var bookCell = sheet.getCurrentCell();
var name = bookCell.getValue();
var person = getPersonFromName(name);
var email = person.email;
*emphasized text*var others = getOtherUsersEmails(email);
removeCellProtection(bookCell);
var protection = bookCell.protect();
SpreadsheetApp.flush();
protection.addEditor(email);
protection.removeEditors(others);
SpreadsheetApp.flush();
}
function removeCellProtection(cell) {
// Remove range protections from specific cell.
var sheet = cell.getSheet();
var protections = sheet.getProtections(SpreadsheetApp.ProtectionType.RANGE);
for (var i = 0; i < protections.length; i++) {
var protection = protections[i];
if (protection.getRange().getA1Notation() == cell.getA1Notation()) {
protection.remove();
SpreadsheetApp.flush();
}
}
}