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Well, if your ISP's DNS server updates regularly, and doesn't need to redirect DNS requests to another DNS server, then the latency involved in DNS resolution would be equal to the latency between you and the ISP. Since this is naturally the lowest latency possible concerning any connection you make with an external IP address using your internet connection, then it makes sense that public DNS servers would not offer improvement over the ISP's ones. However, for less common URLs, a public DNS server may offer lower latencies to resolve the IP address of the domain name requested.babum wroteAnyway, I never felt any improvement whilst using any public DNS over the ISP's ones. (In terms of location of content speed)
Kassem sums it up nicely. :D