You possibly also want to end your benchmark after the output is flushed.

<?php
your_benchmark_start_function();

ob_start ();
for ($i = 0; $i < 5000; $i++)
    echo str_repeat ("your string blablabla bla bla", (rand() % 4) + 1)."<br>\n";

                              <----------
echo your_benchmark_end_function();      |
ob_end_flush (); ------------------------
?>