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... reason being FastCGI protocol has less overhead than HTTP.
1 year, 6 months ago on Why is FastCGI /w Nginx so much faster than Apache /w mod_php?
The bigger issue is the memory footprint. If the server hits its swap, the performance will degrade. Nginx facilitates much larger scale on smaller, more economical hardware by using a smaller, more predictable amount of memory under load.
It's a SWAG, but Nginx might be a little slower because each request has multiple asynchronous events; I think along the lines of send upstream and listen upstream.
What version of Apache were you running? 2.4 claims to be faster than Nginx, but it's only been around for a year.
2 years ago on Why is FastCGI /w Nginx so much faster than Apache /w mod_php?