I just changed nameservers on a bunch of domains during the past few days.
Interestingly, I noticed that from some registars (e.g. Registerfly) the new nameservers propagated quite quickly (some within 2-3 hours), whereas I am still waiting for them to propagate from other registrars such as 1&1 or NetFirms/TuCows even after nearly 3 days!
Does anybody know whether these huge differences between registrars is normal?
You will see that the authoritative TTL on the NS records is 21600 (6 hours).
This means that the longest someone will keep going to the old name servers is 6 hours. So even if the registrar makes the change right away in the root name servers people will may have to wait up to 6 hours to every find the new name server to query.
6 hours is actually quite small.
Some authoritative name servers are cheap and set the TTL to 3 to 5 days (save on queries) to save on bandwidth and processing.
Most authoritative name servers are set anywhere from 1 day to 2 days.
But this is why you may get different response times for domains.
Bottom line: It's not always the registrars fault as the previoud authoritative name server may have had long TTLs on their NS records.