dns - Image domain vs. subdomain -

i'm nearing end of development of site, , offloading images, scripts , fonts second server pool. currently, static pool io.mydomain.com , site mydomain.com (www.* redirects naked domain).

it's been documented* using separate dns lookup static assets improves performance doubles concurrent asset downloads, i'm trying find highest performance method of achieving this?

my question this: dns perspective, better use subdomain (tld lookup, domain lookup, subdomain lookup), apple (images.apple.com), or separate domain (tld lookup , domain lookup) yahoo , microsoft (yimg.com , c.s-microsoft.com)? there of difference between 2 or negligible?

*https://developer.yahoo.com/performance/rules.html

presuming nothing cached, there infinitesimally small improvement brought having same top level domain.

for same top level domain queries queries go

1. query root servers .com. name servers
2. query .com. name servers .example.com. name servers.
3. query .example.com. name servers www.example.com
4. query .example.com. (cached address) name servers io.example.com.

for separate domains be

1. query root servers .com. name servers
2. query .com. name servers .example.com. name servers.
3. query .example.com. name servers www.example.com
4. query .com.(cached address) name servers .xmpl.com. name servers.
5. query .xmpl.com. name servers io.xmpl.com

once first query made - long hadn't set incredibly short expiry - client never need them again.

at best, might shave millisecond first query. after changes nothing.

it isn't remotely worth thinking about! there many other places transiently loose sort of time.