We noticed this when looking at an issue with timeouts in the rss_client block.
The failure we were seeing was caused by the rss_client block calling lib/web.php->break_up_long_words($description, 30) where $description was a string of around 4,600 characters.
break_up_long_words() instantiates a new textlib class and calls $textlib->substr on each character in turn; which in turn performs an iconv_substr on the supplied text.

Effectively, the code is calling:

$strlen = $textlib->strlen($text);

for ($i = 0; $i < $strlen; $i++) {

  $foo += $textlib->substr($text, $i, 1);

}

I'll attach some code which demonstrates the performance compared with mb_string()