PHP script lag (5 secs) when declaring mime type.

Hello, I don't know for sure is this problem is only related to PHP so I hope I've posted in the right list. My problem is that with certain headers my script takes about 5 seconds before sending the page, no matter how small the file I try to load... My server : Centos Apache 2.2 PHP 5.2.9 Here's my code : <?php header('Content-Type: application/x-javascript'); header('Content-Length: '.filesize('test.js')); readfile('test.js'); ?> test.js is only a few lines long, and if I remove the header content type the file loads instantaniously do it's not a problem with readfile. I thought about zlib gzip taking maybe a long time to load but I've changed the compression level from 6 to 1 and the file still has a the same lag. My server responds very fast for eveything else except this script. Do you have an idea what might be causing this lag ? Thanks in advance :)