When to use async false and async true in ajax function in jquery

It’s not relative to performance…

You set async to false, when you need that ajax request to be completed before the browser passes to other codes:

<script>
    // ...
    $.ajax(... async: false ...); // Hey browser! first complete this request, 
                                  // then go for other codes

    $.ajax(...); // Executed after the completion of the previous async:false request.
</script>