PageSpeed Insights / Lighthouse and flushing

0.note.md

The results will be posted in comments

1.flush-then-sleep.php

<?php echo '<p>' . date('Y-m-d H:i:s') . ' pre flush()</p>'; ?>
<?php echo '<p style="font-size:8px;word-wrap:nowrap;">' . str_repeat('padding', 4096) . '</p>'; ?>
<?php flush(); ?>
<?php echo '<p>' . date('Y-m-d H:i:s') . ' pre sleep(5)</p>'; ?>
<?php sleep(5); ?>
<?php echo '<p>' . date('Y-m-d H:i:s') . ' done</p>'; ?>

2.sleep-then-flush.php

<?php ob_start(); ?>
<?php echo '<p>' . date('Y-m-d H:i:s') . ' after ob_start()</p>'; ?>
<?php echo '<p>' . date('Y-m-d H:i:s') . ' pre sleep(5)</p>'; ?>
<?php echo '<p style="font-size:8px;word-wrap:nowrap;">' . str_repeat('padding', 4096) . '</p>'; ?>
<?php sleep(5); ?>
<?php echo '<p>' . date('Y-m-d H:i:s') . ' pre ob_end_flush()</p>'; ?>
<?php ob_end_flush(); ?>
<?php echo '<p>' . date('Y-m-d H:i:s') . ' done</p>'; ?>

3.no-sleep.php

<?php echo '<p style="font-size:8px;word-wrap:nowrap;">' . str_repeat('padding', 4096) . '</p>'; ?>
<?php echo '<p>' . date('Y-m-d H:i:s') . ' done</p>'; ?>

Comment

The PageSpeed Insights result to test 2. Sleep 5 seconds, then flush is very unexpected. Seems like an error. After revisiting the test I noticed I've echo'ed before calling ob_start() so that could have been sent early. But after I corrected the code I still got the same unexpected results. Still some BOM or some other byte can be sent early but it can hardly be called Contentful.

Note: The Lighthouse tests were performed in Vivaldi 3.7.2218.52 but the results are equal to those in Google Chrome 89.0.4389.114.