Having previously failed to get a significant difference in benchmarks between PHP 5.3 and 5.5, I was successful in establishing that the optimizer was producing code which ran slightly faster (about 2% with DokuWiki).
This time I tried ramping up the concurrency to see if PHP could deliver on its performance promises.
ab -n 12000 -c $X http://localhost/src/doku.php
For $X in [10,50,100,200,300,400,500,600,700,800] I got....
For less than 500 concurrent connections, PHP 5.3.3 is slower - but only about 5%. At more than 500 concurrent requests, 5.5.1 is slower!
I'm just no good at this benchmarking business.
Both were running from nginx+php-fpm with the same (common) config. 5.5.1 had full ZOP+ optimization enabled.