If it's bound by anything, it's bound by memory and the database. I would expect the latter to be the bottleneck, if anything. Building an average page requires several trips to the database.

I don't know how aggressively the node cache works in the version of Everything running this site. The performance tweaks in the latest version of the Engine (pre 1.0) involve aggressive caching to avoid extra trips to the database and compiling dynamic content.

It's possible to tune things better, too, by serving images from a different server, by pre-requiring all modules before Apache forks off any kids, and by decreasing the memory requirement of each new kid.

But sharing a pipe with E2 probably slows things down too.