DNS is a distributed system to allow load balancing across multiple dns servers. basically the sa520 is a caching dns proxy. The way it works is:
client wants to connect to something (www.cisco.com for example). The client looks in local dns cache, if it has resolved this recently it will use the locally cached IP to connect. (fastest)
if the local cache has timed out or does not have an entry for www.cisco.com, the client goes to its dns server (sa520), if the site is cached there, the SA sends the dns reply back and then the client goes to the site. If the site is not cached, the sa520 will go to it's resolver (Roadrunner), and RR dns may go to it's resolver, and the process continues until the dns process gets to a server that knows the .com .cisco. and www (these might be 3 different servers)
but, and here's where the local caching comes in to make it faster. if you have multiple devices on the network it is likely someone else has already gone to this site, it is cached locally and the router can return a result without resolving over the internet.
This is more important with web pages that have multiple http connections to multiple sites (facebook icons, ads, twitter links, graphics, etc).
Local caching also puts less of a load on the root internet servers, or googles dns.
for a middle config, you can point your SA520 wan dns settings to 8.8.8.8 (google) and see if it impacts your performance, but it really depends on the mix of dns requests that are happening on your network.
generally local caching on the router pointing to a fast dns is the best mix to reduce traffic and DNS querys, but still respond quickly.
Note, a lot of 'slow internet' issues are really caused by slow dns resolution (along with bad programming of web pages with 10s or hundreds or links on them.
hope it helps,
Dan
