Extra ob_start(""); in phpbb?

I was learning about ob_start and how it can help you with posting more headers without problems, and later on how it could compress the output. Then i thought about the phpbb forum. There already is a ob_start('ob_gzhandler') in a header which is included, and in the phpbb configuration the Enable Gzip compression option. On php.net i read about Gzip en Zlib compression, gzip gives slower response times because it waits for the complete page, and then starts compressing while zlib starts compressing at the first bytes it receives from a page.. parrallel compressing. Therefore gzip produces more cpu load. Then, ignoring the fact that there already is an ob_start in a included header, i put ob_start(""); at the beginning of index.php and watched the response times then. I tried i lot more actually, and ob_start(""); with the "" gives better results as a normal ob_start();. response times, with gzip compression on (in phpbb configuration) AND the extra ob_start(""); at the top of index.php, give a increase of 0.25 avg to 0.18 avg response time. Now, with this large introduction.. how can it be? They don't give usable answers on phpbb.com.. Is it that this extra ob_start("") creates a stacking buffer? And why seems ob_start() to differ from ob_start("") ? (response times mod (puts a line like "phpBB Created this page in 2.902957 seconds : 16 queries executed : GZIP compression disabled" at the bottom): http://www.phpbb.com/mods/downloads/uploads/generation_creation_1.0.0.txt) Thank you very much in advance! Greets, Marvin Vek