Optimize OnEdit() script - google-apps-script

Question

I'm trying to develop a simple script to create a log table to show certain information when a checkbox is marked on one of the Sheets.
The script works correctly (it does what it needs to do) but sometimes works slow. If more than one user 'edits' the Sheet at the same time some entries get lost because the last edit overwrites the previous one. I need to optimize the script as much as possible to avoid losing information.
I think there are 2 possible ways to do it. One is using a function to identify the next available row and the other one is using a countA function within the sheet and get the value from the code (...getRange('whatever').getValue()).
Both of them turned to be slow, so I guess the issue is elsewhere.
I have also tried to use onEdit(e) and getting the active range by e.range but it is also too slow.
Please, find below the code.
function onEdit() {
var ss = SpreadsheetApp.getActive();
var user = Session.getActiveUser().getEmail();
var activeCell = ss.getActiveRange();
var sheet = activeCell.getSheet();
var row = activeCell.getRow();
var column = activeCell.getColumn();
var activeCellValue = activeCell.getValue();
if(!isSupervisor(user) && activeCellValue == true && sheet.getName() != "Activities" && sheet.getName() != "Edit Log" && row >= 10 && column >= 2 && column <= 26 && column % 2 != 0){
var editLog = ss.getSheetByName("Edit Log");
var nextRow = 4 + editLog.getRange('P1').getValue();
var date = new Date();
var language = sheet.getRange(4, column - 1).getValue();
var assignedTo = sheet.getRange(3, column - 1).getValue();
var startHour = sheet.getRange(row, 1).getValue();
var startHourHour = sheet.getRange(row, 27).getValue();
var startHourMinutes = sheet.getRange(row, 28).getValue();
var activity = sheet.getRange(row, column - 1).getValue();
var note = activeCell.getNote();
var generatingID = sheet.getName()+"_"+language+"_"+activity+"_"+startHourHour;
//var shortID = language+"_"+activity+"_"+startHour;
if(!isAlreadyAdded(generatingID, editLog.getRange("A4:A").getValues())) {
editLog.getRange("B1").setValue("Not added");
editLog.getRange("B2").setValue(generatingID);
editLog.getRange(nextRow, 1).setValue(generatingID);
editLog.getRange(nextRow, 3).setValue(date);
editLog.getRange(nextRow, 4).setValue(user);
editLog.getRange(nextRow, 5).setValue(sheet.getName());
editLog.getRange(nextRow, 6).setValue(activeCellValue);
editLog.getRange(nextRow, 7).setValue(language);
editLog.getRange(nextRow, 8).setValue(activity);
editLog.getRange(nextRow, 9).setValue(assignedTo);
editLog.getRange(nextRow, 10).setValue(startHour);
editLog.getRange(nextRow, 13).setValue(note);
} else {
editLog.getRange("B1").setValue("Already added");
editLog.getRange("B2").setValue(generatingID);
}
}
}
function isAlreadyAdded(generatingID, values) {
for(var i = 0; i < values.length; i++) {
if(values[i] == generatingID) {
return true;
}
}
return false;
}
function getFirstEmptyRow(data) {
for(var i = 0; i < data.length; i++) {
if(data[i][0] == null || data[i][0] == '') {
return i;
}
}
}
function isSupervisor(user) {
var supervisors = getSupervisors();
for(var i = 0; i < supervisors.length; i++) {
if(user == supervisors[i]) {
return true;
}
}
return false;
}
function getSupervisors() {
var supervisors = [];
//real values are hidden due to confidential information
supervisors.push("???");
supervisors.push("???");
supervisors.push("???");
supervisors.push("???");
supervisors.push("???");
supervisors.push("???");
supervisors.push("???");
supervisors.push("???");
return supervisors;
}
As previously mentioned, I need to make this script as fast as possible. I can't lose any entry due to the slowness of the script. Do you have any suggestion?
Thanks for your help :)