The "if/then" statements themselves generally won't be a big performance hit, however from my experience kicking in PHP itself can cause a bit of a hit. I would re-write using bigger blocks of PHP and using echos for HTML, like so:
Instead of:
<?php if ($this > $that) { echo 'this'; } ?>
<p>This is not that!</p>
<?php if ($other < $every) { echo 'is'; } ?>
<p>This is not efficient.</p>
/* and so forth... */
You'd want to do this:
<?php
if ($this > $that) { echo 'this'; }
echo '<p>This is not that!</p>';
if ($other < $every) { echo 'is'; }
echo '<p>This is not efficient.</p>';
/* and so forth... */
?>
EDIT An even more efficient approach (as mentioned by MildlyInteresting):
<?php
var $output = '';
if ($this > $that) {
$output .= 'this';
}
$output .= '<p>This is not that!</p>';
if ($other < $every) {
$output .= 'is';
}
$output .= '<p>This is not efficient.</p>';
/* and so forth... */
echo $output;
?>
This is not the best example, but I think you get the idea.
Luke