Why DNS changes take time
ISP resolvers cache an answer for the TTL set on the record. Until it expires, some users still get the old value. With a TTL of 86400, a full update can take a day. Before a migration, lower the TTL to 300 seconds in advance, and raise it again afterwards.
Changed name servers at the registrar? That takes longest: NS records in .com, .ru and other zones have TTLs of up to 48 hours. To check all records from one place, including email security, use the DNS lookup.
Queries run on probes of the open Globalping network through their own ISP resolvers. About 12 free checks an hour.