Genera ran on actual Lisp machines, I feel that that is not a fair comparison.
It's not exactly hidden knowledge that the Intel compilers produce the best code, it's just that they have draconian licensing requirements. Furthermore, that knowledge cannot possibly be lost, because they're still actively developed to this day. I'm interested in the technological specifics of what these older platforms did that the newer ones cannot do, and why the knowledge of how to do them has been lost.
Intel Fortran compiler and their C++ compiler produce the fastest code. Lucid Energize and IBM VisualAge C++ did incremental compilation and had IDE features not available with Free Software C++ compilers and editors/debuggers.
https://www.youtube.com/watch?v=pQQTScuApWk http://www.edm2.com/index.php/VisualAge_C++_4.0_Review