I have a webserver running CentOS 5, with Apache 2 and PHP 5.3. At the moment the server is mainly used for development, so there's really no traffic to talk about. The server houses a couple of Drupal instances, a Mediawiki instance and a few static pages. It makes use of virtual hosts.
The problem is that I get really bad performance with Drupal and Mediawiki. Page execution times are somewhere between 5 and 15 s. The static pages work just fine.
I've enabled mod_forensics and I can see that requests are registered immediately, but then it takes 5-10 s before they are served, so the issue is somehow related to execution on the webserver.
I've installed the Devel module, that provides information about execution, in Drupal and it gives me figures like this.
Executed 59 queries in 62.86 milliseconds. Queries taking longer than 5 ms and queries executed more than once, are highlighted. Page execution time was 6215.2 ms.
So database performance doesn't seem to be an issue.
I've installed eAccelerator and turned on logging, it shows hits for almost all page requests, like this:
EACCELERATOR hit: "/var/www/adqwww/includes/path.inc"
That should mean that the cache is in use, if I understand things right.
Now I've run out of ideas, I need to pinpoint more exactly what is happening, but I really don't know how to do that. Since both Mediawiki and Drupal are affected I assume that this is related to php, but I don't know how to look into it. Any help would be appreciated.
UPDATE: The problem has now been solved (I don't know if there's a proper way to mark the question as solved), it turned out to be a DNS issue, just as suggested below. I didn't understand that the SQL server needed to resolve the connecting web server. Thank's to everyone who took the time to answer this.