Here's my final benchmark results for a real complete page from my site rewritten in two versions :
1. PHP [though mathematical calculations in an extension compiled in C]
2. Everything written in C [though garbage collected]
PHP-fcgi : 100 request/sec.
PHP-fcgi-opcache : 200 req/sec.
C-cgi : 300 req/sec.
C-fcgi : 1000 req/sec.
Notes : Astronomical calculations in PHP all versions were being run through compiled C extensions anyway. Therefore it's not about mathematical calculation.
C-fcgi ran with 5 instances.
If we reduce the instance to just 1 then : 400 req/sec.
And increase to 50 : still 1000 req/sec.
Therefore going all C.