DNS
The domain name system turns a hostname into an address, and a slow or filtered lookup can make a healthy server look slow.
Definitions only — a term explained here is not a service we sell.
DNS resolves a domain name to an address before any connection starts. Slow or filtered resolving happens before the request leaves the device, so a healthy server can look slow.
A stub resolver asks a recursive resolver, which walks from the root to the authoritative server and caches the answer for the record's time to live (TTL).
Why it looks like a slow site
A first lookup adds a round trip before the connection starts. A slow resolver, a long CNAME chain or an expired TTL during a migration can look slow while the server is healthy. It shows up as high TTFB with a fast server.
Answers vary by resolver location and geo-steering, so one local test is weak proof. Resolve from several networks, compare latency and TTL, and query the authoritative servers directly. The China readiness check includes resolution from different vantage points.