I have observed that freshly registered domains propagate much faster than propagation of domain in use.

This is in reference to how much time it takes for me to see site at this domain whose nameservers have been changed.

Is it true or just my imagination? If so, what could be the reason?

It's probably just your cache causing a delay.

You're not the only one who has observed it.

The reason is, most likely, the DNS cache done by the ISP. For a completely new domain there can be no cache, so there's no waiting time related to it, which is why it usually takes minutes for a new domain to "propagate".

I guess you are right becasuse evenafter i change NS on old domains, i keep seeing older site while my friends see new site if they haven't visited same in a couple of days or so.

Nice to know that i am not the only one observing it.