If you want to optimize the parallelization of static resource downloads, you can use subdomains by defining in the vars_<project>.yaml something like this:

# subdomains for static resources
subdomains: ['s1', 's2', 's3', 's4']

Those subdomains must be defined in the DNS and in the Apache vhost. If the application is served on different URL and you want to use the subdomains on only one of them, you can define in the vars_<project>.yaml the following:

# The URL template used to generate the subdomain URL
# %(sub)s will be replaced by the sub domain value.
subdomain_url_template: http://%(sub)s.{host}