<p>We've all read the benchmarks and know the facts - event-based asynchronous network servers are faster than their threaded counterparts.  Think lighttpd or Zeus vs. Apache or IIS. Why is that?</p>

I think event based vs thread based is not the question - it is a nonblocking Multiplexed I/O, Selectable sockets, solution vs thread pool solution.
In the first case you are handling all input that comes in regardless of what is using it- so there is no blocking on the reads- a single 'listener'.  The single listener thread passes data to what can be worker threads of different types- rather than one for each connection.  Again, no blocking on writing any of the data- so the data handler can just run with it separately.  Because this solution is mostly IO reads/writes it doesn't occupy much CPU time- thus your application can take that to do whatever it wants.
In a thread pool solution you have individual threads handling each connection, so they have to share time to context switch in and out- each one 'listening'.  In this solution the CPU + IO ops are in the same thread- which gets a time slice- so you end up waiting on IO ops to complete per thread (blocking) which could traditionally be done without using CPU time.
Google for non-blocking IO for more detail- and you can prob find some comparisons vs. thread pools too.
(if anyone can clarify these points, feel free)