Can't serve multiple files at once

My webserver, Litespeed, refuses to serve multiple files at once for a single IP. For example, if I load a PHP file that, in return, is forcing the download of a 30MB file, that domain will become unresponsive until the download is complete (or canceled). Does anyone know the cause or fix for this?

(And I'm 99% sure Apache and NGINX were doing this for me too)