Hi,

I don't see any reason why it can be slower since the recommended ways to pass params to fastcgi is by using dynamics variables ex: fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;

The way you're setting your variable is static by the website, the value is not dynamically linked to the request. So it might have a sight overhead when starting the web server for parsing the config, instead of having the full config repeated for each virtual host, but overall once the "virtual server" is loaded into memory it's shouldn't be a problem.

Thanks