wp-includes/js/tinymce/tiny_mce_config.php gave problems in my installation. That file took almost 16 seconds to load.
After some debugging I found out that if I comment line 259:
header( 'Content-Length: ' . strlen($content) );
its loadtime was reduced extremely, back to a more normal value.
When I looked at the tiny_mce_config.php alone I could already see al content while it was still loading. So the browser was waiting for content while everything was already available. So it looks like the strlen($content) returns a value that is too large.
The browsers I used to test this were Firefox 3 beta 4, Safari 3.10 and Opera 9.23