Short answer: about 5%
I had a few minutes and wanted to see if changing from Apache + mod_wsgi to Nginx + gunicorn would make the otherwise slow site any faster. It's not this site but another Django site for work (which, by the way, doesn't have to be fast). It's slow because it doesn't cache any of the SQL queries.
# with Apache + mod_wsgi $ ab -n 1000 -c 10 http://thelocaldomain/ ... Requests per second: 39 [#/sec] (mean) ... # Uses about 110 Mb
That's after running multiple times and roughly averaging the requests per seconds.
# with Nginx + guncorn --workers=4 $ ab -n 1000 -c 10 http://thelocaldomain/ ... Requests per second: 41 [#/sec] (mean) ... # uses about 70 Mb
So, if you want to make a site fast forget about how the code is being served until all the slow db I/O is taken care of properly.