BEA Tuxedo (a pre-Java "application server" sort of thing) uses
multi-process model similar to FastCGI, and I've never heard of any
scalability problems with it, other than deadlocks or full table scans
at the DB level. If there is not much DB contention, the application
logic itself scales to tens of CPUs easily.
That's when BEA was a good company and wrote good products. Anyone
who had ever used Tuxedo (I was one) should be downright disgusted at
the "quality" of their app server.
Tuxedo rocked; scaled as much as we could ever ask it, was robust, ..
man, what a good product it was. And I'm not generally kind to
products.