We are considering moving one of our two DNS our clients use to the country where 90% of them live (the other will stay in the US).
Do you think they'll notice an advantage of site loading performance? Even if it's small, we would like to know if going forward with this change will help in any way or if it's pretty useless.
I believe there'll be 50% chance that DNS resolution for the first load would be a bit faster. After that it will be cached in user's dns servers (usually hosted by ISP), later dns lookups will use the cache (of course if you have adequate TTL).
So, I don't think anyone will notice it. But having two dns servers in very different geo locations is very good idea - if one of them will be unavailable (e.g. due to some internet connectivity problem) users should still able to load website.
About "they'll notice" I'm not sure - it's probably a matter of some hundred milliseconds, but anyway, the answer is -
Yes, at least half of DNS lookups will resolve faster. Additionally, DNS resolvers should (at least theoretically) lock on the DNS server that answers faster, thus possibly most lookups from that country will use the closest server. It's a good thing to do.
Edit: It's half of lookups that don't hit cache somewhere. What exact percentage of lookups will hit cache is very dependent - it may be most and may be close to none, depending on TTL you set and on popularity of your domain.
