On a clean server I installed Lenny, I used aptitude to install Lighttpd (1.4.19 a bit old, I know), Mysql, php5-cgi (with php5-apc).
Nothing unusual, nothing strange, no handmade modifications. I think that was pretty standard, but I found (thanks Munin) that MySQL was having big utilization spikes (and so was the server’s load) while admins of a WordPress installation where POSTing things, doing Uploads, etc.
Then I found someone on the Lighttpd forum having a similar issue with FreeBSD. So I tried to change the “server.network-backend” conf from “linux-sendfile” to “writev” and the issue disappeared.
I hope this can help someone else.