Why OpenLiteSpeed is so much faster than Nginx?

Why OpenLiteSpeed is so much faster than Nginx?

I am already aware of Ningx using event driven architecture. And
OpenLiteSpeed uses same architecture. But when we are comparing raw php
performance for a real world Script like Vbulletin, why Ningx + php fpm
suffers badly compared to OpenLitespeed + LSAPI.
Now, finally OpenLiteSpeed is opensource, can anyone look into its code
and tell me architecture different between these two servers and how their
performance varies so drastically?
Benchmark : http://i.stack.imgur.com/d0QFG.png
Some thoughts
FPM is stuck with the overhead of latency and IPC delay from its CGI legacy.