DNS Resolution Chain
The sequence of lookups from browser cache through OS cache, recursive resolver, root, TLD and authoritative server that turns a name into an address.
Each layer caches, so most lookups cost nothing. A cold lookup walks the chain: the recursive resolver asks a root server for the TLD's nameservers, the TLD server for the domain's authoritative nameservers, and the authoritative server for the record.
Latency ranges from zero (cached) to 20–120 ms (cold, several round trips).
Two things this explains. CNAME chains multiply lookups — each hop is another resolution, so a chain of four is four sequential round trips before any connection begins. And the authoritative server is where traffic steering happens: latency-based, geo-based and weighted routing are decisions made at that last step, which is why DNS is a traffic-management tool and not only a directory.