If your subdomain will be located on the same server, then no acceleration you get.
Acceleration can be if put in front of apache and nginx catalogue with the statics give by means of nginx without recourse to apache.
A must read
lib.clodo.ru/web-%D0%A1%D0%B5%D1%80%D0%B2%D0%B5%D1...
Or anything according to "nginx apache".
Read about: Parallelization of content. - Buck.Walter commented on July 9th 19 at 10:54