Dear brother I have 6 DA servers 1 DA server from Singapore if I link all servers with DA Multi Server Setup, If someone visit website from Singapore and that website data hosted on USA DC it will improve DNS latency or not ?you just need to setup DNS servers in nearest to user DCs with good connectivity.
usually each DC provide test files/links on their websites to check latency/speed etc.
I think this is not good to create Seprate nameservers for each DC, we can do it for low level with some servers once we setup many servers create Seprate nameservers its not good idea, always client message is what is my server nameserver.if your DNS configured just like: ns1.your.host with 6 A-records for 6 different DCs/servers - so your clients will randomly ask DNS server, so sometimes they will have answer from nearest servers sometimes from far away one. I don't have experience how to configure "weighted" dns to make it response to user from nearest server - you can google how to do it. I prefer simplest (for me) config - for each DC I have own NSs. So if client's website oriented to Europe - I put his website to European DC with local NSs. Same for USA and so on.