After getting covered with benchmark and stumbling over time-outs reported by http_load it was time to look a bit deeper into the problem.
http://www.lighttpd.net/2007/3/5/thread-starvation talks about how you can reduce the probability of time outs reported to the user. With the help of a new timing infrastructure you can now track the time spent a different stages of a request. With the help of gnuplot you can get a feeling of where the time is spent.
To make the long story short: If you use one of the async-io backends in 1.5.0 you want to set the server.max-read-threads to twice the number of disks.